Not a plugin but After Effects has to be the best value for money tool to be using in conjunction with Vegas. A wealth of excellent tutorials and projects available for free.
On the upside the plugins from Boris, Red Giant and New Blue etc all work in it and better. It can take a lot of the load off Vegas which does get very 'sticky' with lots of plugins.
On the downside a fairly steep learning curve. Nothing as bad as trying to learn a full 3D application but certainly you get out of it what you put into it and you need to use it fairly regularly I find or you can get rusty. It is not intuitive in many ways until you get your head right around it.
As others have said though it really depends on what you want to do. Vegas has most if not all of the tools to create excellent video. Well shot well edited video with great sound beats the pants of anything you can achieve with plugins. If your outcomes are lacking something I'd always start looking at your original vision and sound before anything you can do after it is shot and recorded. My apologies if you already know this and I'm preaching to the converted :)
I would cast my vote for Pixelan - I use them constantly, but it really gets back to what you want to use them for.
I also use Excalibur and UltimateS on every edit.
Oh and don't forget the Veggie Tool kit (host of useful plugins, including a very flexible multiple rendering plugin), the Neat Video Noise reduction (if you need to eliminate grain), Plural eyes (if you need to quickly synchronise multiple cameras), ProDad Mercalli (if you need to stabilize shaky footage). The list (like the beat) goes on...
How come no one mentioned Mike Crash's FREE plugins?
I was waiting for you to do it! - And yes, Mike's Auto Levels "Full Colour Correction with film levels" simply marvellous! - It is based on Auto Levels filter for Virtual Dub from Livio Cicala.
"Does Frameserver work reliably in version 9 yet?" - Yes. I have it working on an "unrendered" - Frameserving - AVI with a SONY Newsprint FX which is currently varying from LARGE newsprint Offset Colour to SMALL Offset Colour Dots and STRAIGHT into Particle Illusion as a BG with explosions all over the shop. No problems. As slippery as snot trying to stand upright on a well-ripe rancid banana peel! - It works! The unrendered 9 secs AVI is er . . 1.25mb.
"How about 64-bit?" - No. Satish has put out a Global Call for any programmers to get with him on this.