Yep, that's what I did... a new shared account to use it for VPS domains.
The advantage, it's that you have a high chance you get new MySQL servers asigned, so you could choose which one to use for both hosting plans.
In my case, I had to move all the databases from one mysql server to the other, as one of them was having an awful performance.
Take a look at these graphs:
The last two graphs show both of those MySQL servers. Both of 'em have the same uptime and look at number of queries server, and the QPS (Queries Per Second Average).
Server 1 is being taken to his knees with a huge amount of workload, meanwhile Server 2 is pretty more relaxed.
Obviously, I suppose both are using similar hardware, as I noticed the difference a lot by switching database servers... and all of 'em are in the same LAN (I guess so), so there's no penalty for swapping
I was writing this explanation at that blog, but I only have it at draft status, as my english is not my primary language and I've to review it a few times.