Actually, that kind of banh cuon is not a hot dish and should never be served that way. When it's hot, it's more porous, so it soaks up and dilutes the flavor of the fish sauce, requiring you to use more sauce than you should. It would also cook the lettuce and cucumber (which should be fresh--the only veggie that should be cooked is the sprouts, which is usually steamed) and change the taste and texture of the banh cuon itself, which for most people goes unnoticed, but for me is quite an abomination. Vietnamese food, even a simple dish, has so many dimensions and is very very specific!!