I would like to say you're ok but from my point of view I'd be lieing.
I had a real problem with these o-rings.
I had air bubbles in fuel line at high rpms and at certain times they were high that the engine stumbled nasty.
I changed the o-ring with the same size but fatter and the thermostat valve (t thingie) and since then had NO PROBLEM and no visible bubble only some little bubbles which can be seen using a magnifier glass ONLY.
But how expensive can such an o-ring be??! And availability should be no problem there.. starting from vw dealer and other automotive centers you will definitely find one.
Size? Measure the size on the new filter (or old one - or one in a store without buying it) and then search for such an o-ring.
You will manage in the end.
MY RECOMMENDATION: Put a NEW o-ring there ... maybe even a new thermostat valve.
You gotta be careful with the built-in o-ring. When I changed my fuel filter, I was kind of rough putting the "T" thing back on and pushed the o-ring off its seat a little. This caused some rough start ups and idling. Reseating the o-ring solved this. Lesson learned