Hi CaryDave P. had some really excellent suggestions for you. Also, in Mike Goulian's book - I believe Advanced Aerobatics - I wrote about vertical rolls, as did he, so I suggest you get that
Whatever you are flying it takes loads of fuel and practice to get vertical rolls just right. It's easy to let the airplane go on its back as it gets slow and easy to let a wing drop - specifically the left one. As you pull into the vertical, look straight ahead and keep the airplane tracking straight. Keep the ailerons straight (it's easy to add a bit inadvertently), add a touch of right rudder for P factor and to keep the right wing down and just practice getting to vertical. Take it all one step at a time, get critiquing from someone qualified on the road and you'll get there.
Patty