As an owner of Ascent is, my personal opinion would be that no, that wouldn't be a significant step for the money. The Vantage sells for roughly what the Ascent i did new. Assuming that your friend will be selling the Ascents, he/she may get somewhere in the range of $2.5-2.8K US for them, then have to plunk down an additional $2-3K for new Vantages.
The Vantages have the new panels, which will sound somewhat better. I personally don't think that the bass is significantly different, and the panels are not a big-step improvement over the Ascent. The Vantage does have powered bass drivers, but if you already have sufficient amplification, I don't see that as a factor.
The Summit
would be a big-step improvement in both regards. One advantage of the Vantage is it's smaller size/footprint, and it has the new cabinet style, all of which can be contributing factors for the decision.
If the real issue is simply with bass, adding a sub makes a significant difference. I added a Depth to my system after about six months (I auditioned with and without one, and knew I'd get one eventually). For me, the Depth "cured" the lack of bass and punch, and still blends well with the panels. You do have to dial them in for your particular setup. A new Depth is around $1.8K US. There is one on Audiogon now listed at $1.35K new-in-box.
http://cls.audiogon.com/cgi-bin/cls.pl?homesubw&1150298931