I haven't been in a Vista since the 2004 Tampa Supershow, so don't remember much about interior details. In other Winnebago models with bunks flanked by sofa and dinette, I've seen the brackets for the ladder mounted on the hard lower edge of the section of the bed that pulls out to cover the cab opening. If the hard base has insufficient width, I would probably fabricated a filler piece for that opening, maybe 2x2 hardwood, and attach to that. That would be a DIY ladder stop.
Winnebago often does not offer ladders for models where the dinette is in a position to be used to climb into the bunk. Instead they beef up the dinette seat back to serve as a step. It can be a long way up from there.
Tom Test
Itasca Spirit 29B