If I'm HIV positive and taking biktarvy is it ok and a good idea to take vitamin c pills too?

Vitamin C does not interact with Biktarvy, but do inform your Dr. of any new medications or supplements you want to add.

https://www.drugs.com/interactions-check.php?drug_list=3891-18772,238-3823

"Many drugs can interact with bictegravir, emtricitabine, and tenofovir. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using."

https://www.drugs.com/biktarvy.html#interactions