Can you trust a negative result of a Snap Lyme test in a previously vaccinated dog?
If the dog is limping would you proceed and do a Western Blot test?

the snap test shows exposure to Lyme and the antibody production due to the exposure. It doesn't indicate an active infection unless there are clinical signs as well.

If it is early in the disease, ie with in the first 10 days after the tick bite, it will be negative.

You can also do a C6 protein test through Idexx that is a little more accurate than the snap test.

The snap test should not show a positive if the dog has been vaccinated for Lyme.