Yes, you use either in order to relax the vinyl and allow it to lay correctly. My problem was that once I got the vinyl heated it would either double over on itself because it got too soft, or before I put the hair dryer down it got too cool and wouldn't conform. Either way, once I would get it to stick it would cool and peel itself back off, hence the relief cuts I made.
I should also add that I applied this on a cold winter day, after dark, in my house which I keep at like 62. Plus the furniture was cold.
I would certainly suggest doing this on a hot, sunny summer day in direct sunlight to allow the vinyl to relax as much as possible. In that case you probably wouldn't need the heat gun.