He'd have cleaned out his division about as thoroughly as any heavyweight in history. He'd have beaten every man worth beating in a decade of division history. He'd have probably a top 5 or 6 win streak in heavyweight history. He'd finally have a win against a true ATG heavyweight. Damn right top 10. Probably 9 or 10.
I think it might be enough to barely boost him into the 10.. But it wouldn't be a win of enormous magnitude given that both men are past it, and Vitali in particular is ancient.
Number 4 - only behind Ali, Lenny Lewis And BIG George. (if he gets a few more years of title defenses after unifying)
He'd probably have to nudge inside the top 10 (cosidering Vitali is old), tho alot of people will still be very reluctant to put him inside their top 10. I think Liston can give way and Wlad can take the number 10 spot.
It would depend on how he beat him, and how good Vitali would be on the night. If they fought tomorrow, the win would be somewhat tarnished due to Vitali clearly having seen better days and that would not be enough to propel him much further up any rankings, I think. A few years ago...now that's a different scenario altogether. It would be harder (A lot harder) for me to continually question his overall legacy/ability with a quality win like that.