Can I very occasionally have a little edible or use vaporizer pen after gastric bypass surgery ?

Smoking medical marijuana would not cause smoke to get into your pouch. It's just that smoking in general isn't good for you. However, there is no evidence that says smoking or eating marijuana causes any specific problems in gastric bypass patients.

Although there are no studies that make a direct comparison between marijuana use and GBS, there are studies that show that smoking/vaping has been linked to poor recovery of post-operative wounds due to reduced blood flow at the capillary level that does not allow adequate circulation and oxygenation of the tissue to heal properly.
This makes the tissue prone to poor healing and possible infections.

In case of edibles, this should always be consulted with the doctor that following your post operative diets, and taking in mind about all the changes and side effects or interactions these could have with your follow up supplements or post operative medications