Once upon the time I created "rinse" which is an RPM-using debootstrap-like tool.
I've handed that off now, but the project is still very much alive:
Re:CentOS 7 released on x86_64
by sliverstorm on Monday July 07, 2014 @09:17PM
(#8002571)
BuyVM, $15/year
The good news is BuyVM just announced they are preparing a CentOS 7 image, so the installer RAM limit may not matter.
You are right that 128mb is a bit tight, I have problems with some services that just gobble memory. Apache+PHP are terrible in that regard.
There used to be a Fedora equivalent called febootstrap, but it looks like it's mutated a bit since then: http://people.redhat.com/~rjones/supermin/
You may be able to work around it that way.
Out of interest, what machine are you using CentOS 6 on? I find 128mb kinda tight - although I've recently found a VPS provider who's managed to get Debian Jessie working in 96!