Does it have spark?  You say "no sign of ignition" and then say "vapor
lock."  I assume the engine is cranking if you are saying this.

Are you absolutely certain that the battery is OK?  Did you check each cell
with a hydrometer after it has set for a while?  I had a situation on my '97
where sometimes it would start and sometimes it would not.  It turned out
that two of my battery's cells were a bit weak.  I was sure the battery was
not the problem.  What I do think was the problem was a dirty connection on
the crankcase position sensor which on my particular engine (4.0 V6 OHC) is
in the front below the crankshaft.  I cleaned the connection and replaced
the battery and the problem is gone.  I did not have spark but the coil
checked out OK and the voltage to the coil was OK too.  So, either the
computer would not allow ignition because of a low voltage situation or it
would not allow it because it did not know the crankcase position.  Two easy
things to check anyway...

Good luck.

Have you tried cleaning the battery terminals?