I don't know which vest you have, but HillRom will send you a foam thing you can put between your child's shirt and the vest to help cushion the area around the gtube if she is experiencing discomfort. When she first had hers placed we folded a small towel (hand towel sized) and placed it over the site under the vest and this helped.
Emily occasionally has discomfort around her tube area- at the moment she has a granuloma we are treating to try and shrink it. I would investigate to see if there is anything going on that could be causing pain- granuloma, a lot of oozing that could be causing irritation when the vest rubs against it (in which case make sure the site is clean and dry before starting- this is another thing Emily is picky about), is her button a really tight fit right now (possibly she needs a bigger/longer one?) I'd also never do the vest on a really full stomach or while she has a feed running.