Will vaping marijuana slow the or increase growth of my grafts after my hair transplant?

I am not aware of any data linking marijuana use(vaped or otherwise) to hair loss.As you pointed out,nicotine and other by-products in cigarettes can decrease blood flow and contribute to hair loss.

There is no data regarding how smoking or vaping marijuana will affect the results of your surgery.  If you're worried, consult with your doctor.

There aren't any studies done on this so I would wait a month after your transplant- at this point your transplanted hairs are essentially the same as your normal hair, so it's very unlikely it will affect the transplants any differently than your normal hair at that point.