I don't believe statements like:
1. It will evolve
2. It is customizable
3. It is production quality
4. We are customer oriented
5. It is user friendly
6. We can fix any problem you may have after implementation
7. We will help you with implementation problems
Tactics I used:
1. set measurable benchmarks for stuff that are important, and construct tests to validate their claim
2. Don't expect any future enhancement. Assume what you see is what you will get, ever.
3. Put specific service requirements, in details, of what you expect from the vendor, like consulting, training, etc. For example, don't just ask for hours of training, but specifically saying what kind of person will be trained, and what is the resultant skill level you expect, with tests to verify the claim, plus penalty clauses, including time you staff will spend in unsatisfactory training.
4. Prepare for the wors[t], have a plan B if stuff fails.
5. Don't sign anything until tests are done and you are completely satisfied with the results, or have contracts (with penalty clauses) from the vendor to rectify any deficiency.
I have to confess that I did drive a few vendors in to bankrupcy that they deserve.