I came across this water heater vent pipe connected to a older style gas radiator vent in this basement. I’m am looking for further explanation if this is ok. It really didnt seem correct but this is the first time I’ve seen this set up. Any input is appreciated.
You have what looks like 2 gas appliances with a shared vent. Where does that vent terminate?
Look up the venting standards for natural draft gas appliances (if the radiator is such a configuration). Vent type, length, slope, vertical or horizontal termination, etc.
And what is that flex material that passes through the exterior wall?
In general, two water heaters or other gas appliances can share a vent if it’s done properly. That setup looks to lack proper slope in places but it’s hard to tell from the pictures.
Do you have more detailed photos on the radiator thing?
Is it really gas fired? How does condensate water drain out of it, since the flue is not at the top?
It sure looks like a steam radiator from most angles.
Is the pipe on the wall a gas line?
The flue sharing itself, is not a problem, if slopes are good.
A very old clow gas heater. Have it inspected by a qualified HVAC prior use. Parts may not be available to service it. Sometimes you can sell them for the price of a new boiler/furnace 
Time to upgrade it!
