While it is possible to run a different kernel for each of the VPS that are running on a Xen VPS, that might not be the choice of your host, for many reasons. We made that choice too, because we think it's more easy to maintain this way.
Now, even though your kernel is 64 bits, that doesn't prevent you from running an operating system in 32 bits. There are few cases where there are some small issues, but it's kind of rare, and this should always be considered as a bug.
Last, why on earth do you want to run in 32 bits? Especially in CentOS, you have ALL of the libraries that are available in both 32 and 64 bits if you are running a 64 bits OS. So there are absolutely zero reasons why you would like to run in 32 bits, unless you really like things to be slower. Even if you are running some kind of proprietary software that is in 32 bits mode, you can still have all the rest of the operating system running in 64 bits, that's not an issue. Just the libs that it uses are needed to be there, that's it.
I hope that helps,
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