Vault

My vote is yes it is worth the time and effort to install and use the Vault.
Everyone can use common parts.
Content Center is very valuable
Search makes the Vault worth it just by itself.
Backups in a vault have been a life saver for me. I know all your engineers make diligent Backups every night before leaving for the day; ours did not and have lost work.
It is easier to learn the system with three workers, than wait till you have 9 and decide to install and train everyone at the same time.
When we first started SolidWorks back in 1998, using SmarTeam we could only afford 3 seats of SmarTeam. Our Engineers worked on a network drive, without a Vault. The drafters would move their completed projects into the SmarTeam Vault. Then when SolidWorks came with PDM Vault we started using it. What a nightmare getting the entire project list off the network and into the Vault.
If you have the power to get this done great, if not get management to back you with using a Vault.



Standing
AutoDesk Inventor Router 2011
SolidWorks Pro 2009 x64, SP3.0, PDMWorks Workgroup, SolidWorks BOM,
HP xw8600, 64-bit Windows Vista Business, Service Pack 1
Intel Xeon CPU, 3.00 GHz, 16 GB RAM, Virtual memory 166682 MB, nVidia Quadro FX 4600

We implemented vault recently (~2 years), also with 3 users.

I am still torn on if it's worth it.

Without vault, Inventor is utterly useless in a multi-user environment. You will lose part links constantly unless your guys are absolutely fastidious in their procedures.

With vault, that is improved. However it doesn't have the slightest idea how to handle iParts/iAssemblies. If you make significant use of these (sometimes required depending on your work-flow), it will be constantly asking you to check in/out parts that never changed. This causes accidental rollbacks to previous versions, and sometimes loss of iPart members because of this. It has been borked since V10, so I dont see this being fixed anytime soon.

There is also a definite learning curve, doubly so if your guys are not already pro inventor users.

If you all work on the same project then it will be very useful. You don't need it with experienced users but it is still good once you get used to it. Have worked on the same project with junior drafters, without vault there were the occasional problem, more work for the manager to keep track of what each user is doing.
Implementation is easier than you think, there are some good instructional guides around. Copy design is a nice tool but not really quicker than design assistant until they improve their renaming ability.
Only downside is training users if there are many changes to the drafting team. Stand alone projects are much more flexible and I have primarily been using that workflow for years without problems. That said, I would still recommend using Vault if your companies data management will allow it.