Parrots do not need to be "dewormed" at all, nor do they need any type of anti-parasitic medications, vaccinations, etc. at all unless they actively have some type of parasitic infestation. Keeping parrots as pets are not at all like keeping dogs or cats. In fact, most anti-parasitic medications and "dewormers" are actually quite toxic/poisonous to birds, and ONLY need to be administered if your Cockatiel or any other species of parrot shows an active infestation with them, which you would have to have diagnosed by an Avian Vet doing a Fecal Culture/microscopy...But it's extremely rare for a parrot to have an infestation with any type of worms, even if they come from a horrible environment. Parrots are not typically infested with any type of parasites except for certain types of mites, such as "feather-mites", but they must be exposed to mites to contract them, which would mean they have to be exposed to other birds that actively have them. So parasites/worms are nothing that you need to really be concerned about at all with your pet parrots, the same goes with vaccinations.
You should fine either a Certified Avian Vet or an Avian Specialist Vet that is local to you, and you should take your bird at least once a year for an annual "Wellness Exam" just to make sure that your bird is healthy and everything is okay. it's important that you only do this with an Avian Specialist Vet so that they not only know what they're doing, but they know what they're looking for and how to properly do a blood-draw from their neck. During a typical yearly "Wellness Exam", the Avian Vet will do a complete physical/visual exam, looking at your bird's weight and body condition, looking at their feathers to make sure they are growing in properly and are healthy, and that there are no signs of feather-mites, and then they should take a Fecal sample which they will look at under the microscope and do a gram-stain on to check for any Gastrointestinal infections both Bacterial and Fungal (yeast), and to check to make sure that there is an adequate amount of healthy Bacteria present in their GI Tract. They may also send a Fecal sample out to a laboratory to allow the culture to grow-out on a plate over a few days, just to make sure that there is no harmful bacteria or fungi present that isn't visible under the microscope...Then they should do a quick blood-draw from their neck, which should not require any type of sedation or anesthesia, they simply hold them in a towel and quickly draw the blood from the vein in their neck, and the whole thing takes 30 seconds and does no harm to the bird at all...Then they send the blood sample out for regular, routine, baseline blood-work, just like they do for people regularly. They will check for signs of infection, anemia, nutritional deficiencies, and then also their liver and kidney functions. You should get the results in a few days. They should also counsel you about your bird's regular, daily diet, daily exercise, enrichment activities such as foraging, etc. That's it. No vaccinations necessary, and NEVER any "de-worming" or any anti-parasitic treatments unless they see an active infestation, which is extremely rare in any bird, and if they do have a parasite it's extremely important that ONLY an Avian Vet treats your bird, because it's not like "de-worming" a dog or cat, they use single injections of very specific anti-parasitic medications that you cannot obtain in a pet shop or other store, because birds are very, very sensitive to pretty much everything, especially any type of poisons...But again, it's extremely rare that a bird/parrot be infected by any type of parasites, and almost never, ever effected by any types of worms. In 39 years I've not ever seen a bird with any types of worms.
***As far as a "vitamin" supplement, you shouldn't need to give your bird any type of vitamin supplement at all as long as their regular, daily diet is healthy and varied. It's extremely important that you NEVER, EVER put anything at all into your bird's drinking water!!!! You often see "vitamin drops" for pet birds in grocery stores and at pet shops that you are supposed to put in their drinking water. Not only are these "vitamin drops" unnecessary, they are actually extremely dangerous and can make your bird extremely sick or worse. First of all, anything that you put into your bird's drinking water such as a vitamin supplement or other nutritional/health supplement, or any type of medication such as an antibiotic powder, etc. immediately contaminates their drinking water and can make them very, very sick. Both bacteria and fungi (yeast) start growing in their water as soon as you put any vitamins, medications, etc. into it, and your bird then drinks the bacteria and fungi, and often with develop severe GI infections, among other issues. Secondly, you should never give your bird any type of vitamin or other nutritional supplement or any medication in their drinking water because you have absolutely no idea what dose of these things they will be getting. Birds don't drink much water throughout the day, so giving them any medications that way is typically futile; any and all medications should be prescribed by only an Avian Vet, and they should be given to your bird either directly into their beak/mouth with an oral syringe/eye dropper, or by injection from the Avian Vet. Birds are very small and doses of both medications and nutritional supplements must be given in very specific doses, otherwise just a tiny bit too much can cause them to overdose.
With those cheap, liquid "vitamin drops" that you put in their water or onto their food, the doses of each vitamin/mineral is purposely made many, many times higher than the bird should have, as they are trying to ensure that they will get "enough" of each vitamin/mineral by drinking just a tiny bit of water...But it's extremely easy for your bird to overdose on the vitamins and/or minerals and suffer "vitamin-toxicity", metal-poisoning, etc. from them. And again, if your bird's regular, daily diet is adequate, then your bird doesn't need any type of vitamin supplement to begin with. Only your Certified Avian Vet after doing blood-work and diagnosing a nutritional deficiency should prescribe any and all types of vitamin/mineral/nutritional supplements, and they are typically given by a single injection.
***What is your Cockatiel's regular, daily diet?
