LaO always has some good advice. But just to help "muddy up the water", I was reading one of my many gun rags that I subscribe to....and they had an article about how you should shoot from a bench.
Well, other than the usual "Don't lay your barrel on the front rest "dummy" lay the forend on it" kind of stuff" they had some really good information in the article. One of the items they mentioned about vertical stringing was using a rear sand-bag. They said if you do use a rear sand bag (which I do all the time) and you find yourself "pinching" the bag with your "free hand" (in my case that would be my left hand as I am a right handed shooter) you may find that your shots will tend to string vertically. This is because when you pull the trigger, your left hand may not keep the same tension on the bag, and the gun will shift slightly, causing the slight vertical string.
So.....to "fix" this, when you set up, make sure the rear of your gun is SOLID on the bag.....or use a holder such as the Lead Sled or one of those "Wrap Bag" type of gun rest.
FWIW.
Dave