I'm trying to diagnose if my problem is vapor lock or something else. Once the summer temperatures hit, my 1989 300SE will just stall while driving. Usually happens within the first 5 minutes or after a refuel. I sit for a few & try to re-start. It may take about 5 minutes for the car to restart. Also note, that the car does not get peppy until it's running for about 5 minutes or driven a short distance. Is this vapor lock or something else? How can I solve?

Well, Neo, the classic vapor lock (mechanical pump creates low pressure in hot suction line and fuel percolates) really doesn't apply to these cars, since they are pressurized from a rear fuel pump with a recirculating fuel line. That being said, the fuel systems are pretty complex - after 20 years I have yet to discover why my 300SE chugs and stumbles for a few seconds after a heat soak (kinda like the old mechanical fuel pump vapor-lock symptoms, Huh).

Your problem kinda sounds like a partially clogged fuel filter, or a weak fuel pump. If the filter hasn't been replaced in a while, that would be the easiest place to start. Also, there is a small filter where the fuel line connects to the fuel distributor head.
After that, you may need to get into checking the fuel enrichment closed loop circuit, which will require a Sears multimeter with a duty cycle readout, plugged into the round diag connector (lots of good info herein on that subject). This will tell you how the EHA, air valve, O2 sensor are working.
If the system seems to work okay until it warms up and goes into closed loop, then this may be the problem
Pressure checks require a specific gauge set to check lower and upper chamber pressures, regulator pressure, accumulator, internal leakage, etc.

So, try the filter first, then do a search of some of Steve Brotherton's articles on lambda control diagnosis.

DG