Do I need to be worried about an
explosion from these vapors? What sort of concentration is required
before the flame will make a big 'poof.'

In general, worry about the hazard from you breathing it, long before
you need to worry about explosion hazards.

The crucial factor is the concentration, and whether this is in a range
that forms an explosive mixture. For typical workshop use, this just
doesn't happen. If you're insistent on blowing yourself up, best way
to arrange it is to either knock the can over (suddenly raising the
concentration far beyond what you expected) or to have an ignition
source close to where you're working (concentration will be highest
around the can, or a large freshly glued/painted area)..

The significant concentration varies, depending on the chemical in
question. For our workshop solvents, you need a concentration of at
least 1% to form an explosive atmosphere (unless you're using some
plutonium and hydrazine process for staining cherry). This is
unthinkable in the workshop at large, but it's easily done near to a
pool of liquid solvent. So keep a good physical separation between
your solvents and your ignition sources ! This includes low-mounted
pilot lights and heavy vapours (like propane) that will concentrate
either on the floor, or below the ceiling.

There's a handy list of flamable solvents he
http://ptcl.chem.ox.ac.uk/MSDS/lowflashpoint.html
This lists the explosive limits, but it also lists the flashpoints.
Flashpoint is perhaps more useful here - if you're using a liquid with
a low flashpoint (around room temperature) then the vapour above a pool
of it will be an explosive mixture. This is where the definitions of
"flammable" and "highly flammable" come from - "Highly flammable" will
do this at room temperature, "Flammable" will do it with just a little
warmth (like a nice hot cup of tea)

Paint solvents are generally flammable. Glue solvents are often highly
flammable. be careful with paint, but be _really_ careful with solvent
based glues. The "construction adhesive" in mastic-gun tubes has an
infamous reputation for this - something like 75% of vapour explosion
accidents in construction are caused by this - not petrol, not paint
thinners. If toluene is an ingredient, be wary.

need to attach oak skins to kitchen cabinet sides.

This is called "veneer" and you do it with hide glue (ideally hot, but
the cold stuff is useful too). Now I'm a hide glue fan because I still
think it's the best and easiest way, but I appreciate others will have
their favourites. However anyone will tell you that doing this with
contact cement is a royal pain to do, let alone the vapour hazard.

Oh, and vapour exhaust fans need to be of a motor design that doesn't
use sparking brushgear ! (most are OK)