As far as the VPS goes, you've been in business for 6 years and I'm pretty sure your familiar with the technology on how to operate them, if not, Google has a great search tool to help your staff train and learn. Here's my list of reasons for VPS vs. Shared hosting account:

1. Root/SSH Access: This enables them to configure specific on the server to fit their needs and carry specific tests/functions on their own as they build.

2. RMS: If your RMS goes down then all the sites utilizing see more that specific RMS will lose many of the features that pull the members to them such as the Video/Audio Chat & Messengers, Boards and Video Comments (for the D7 sites).

3. phpBB3/IONCube/VBulletin: You have these shared servers configured to run Dolphin which is great, however when someone wishes to integrate something that does not run well in a dolphin environment into the site this becomes difficult to impossible for them to do. Whether it's adding IONCube or resetting the mbStrings to a manner that disables Orca but promotes the running of other software, you can't do it as it'll jeopardize the other sites on your servers.

4. Security: You can not possibly argue that your shared servers are anywhere near as secure as a VPS or Dedicated Server. On a VPS or Ded once a hacker gets in they can only nail what that one customer has on their server, but should a hacker pop onto a shared, they get the run of however many customers' (notice the plural) sites that are on the server. You can't even give out the SSH Access on these servers because a noobie could error accidentally and bring down all the sites in one fell swoop. A year or so ago a host had an issue with a site owner uploading bad mods (they contained a virus) to a shared server environment and it brought down hundreds of accounts as it moved through the entire machine. No, I'm not going to mention who it was.

5. Hard Walls: Nice hard partition between accts, need I say more.

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