If i gift CS:GO to my smurf and get vacced on the smurf. Will it infect my main?
I don't think so. I also don't think you will be able to gift csgo again if you get vacced on your smurf. If you link the same phone from your main onto your smurf and get vacced, both accounts get vacced aswell.
No since you're just gifting the game your main won't get banned. If you buy someone a gift and they get banned it doesn't really make any sense that you should be banned too. Make sure you don't link the phone numbers though, because then I believe you can get banned on all accounts associated with the phone number.