Can you make vegan garlic bread, if so how does the use of margarine effect taste and texture?
Margarine will  not impact the texture, and probably will not alter the end result dramatically.  It, of course, is not butter, so as long as the flavor is fine with you.  But, you also don't need butter or margarine. You can use olive oil.  Sweat garlic in olive oil.  Proceed from that point.