Aside from WWI planes, I've flown some high performance planes in legit dogfights, as pilot and as passenger. I've also taken several aerospace physiology courses. I'm not an expert, but I do have a well above average education in it as well as practical experience in a variety of aircraft.
For a pilot withstand G forces, there are a several components that play a factor. Natural physiology, anti-G strain, G-suit, pilot position in the aircraft (for WWI and WWII aircraft, pilot position does not vary enough to make a difference).
Natural physiology is that the average human sitting in a chair can sustain about 3.5 Gs without blacking out and without performing any anti-G strain.
As G forces increase above that, an average person will need to perform an anti-G strain. A sustained G force of 9 Gs is an average maximum. A G-suit gives an extra G of tolerance.
The WWI Fokker Dr.I could withstand about 6.5 Gs, obviously highly dependent upon construction quality. This is probably a good average for most WWI aircraft, and the reason I mention that is because we aren't going to see 10+ Gs from a WWI aircraft. We are really talking about G force physiology under 6 Gs.
WwI aircraft cannot sustain 6Gs. They can't even sustain 4Gs. In a level turn, max sustained Gs is probably no more than 2, which would be a 60 degree banked turn. (I'll rig up my G meter for my next flight).
Momentary (3 seconds or less) G force upto 9 Gs (indeed to double check that) wont even cause a gray out.
My experience in WWI aircraft is that it is impossible to sustain 3.5 Gs for more than 180 degrees of turn. I havent entered a turn from more than about 115 mph, but the turns I have done have all been around 2Gs. My resting G tolerance is right about 3.5, and I haven't had even the slightest gray out in my vision.
When I was dogfighting in high performance planes, the typical dogfight breaks down into about 4 G sustained turning. Yes, it is tiring after 5 minutes, but a modest G strain will keep the gray away. If you had a fight at 4Gs and then you had to get on it at 6 Gs or more, it could be challenging to keep the gray away. I would not hesitate to say that I could spend 10 minutes or more at 4Gs fighting though.
In WWI, I'm not sure if they knew about anti-G strain. In this case, an average pilot would begin to gray out at 3.5 Gs and could black out or suffer G-LOC at anything more than that.
Wing vapor trails on a WWI aircraft are a bit of a stretch in my opinion. Vapor trails come from high velocity air creating low pressure vortices off the wing tips. Greatest low pressure occurs with high angles of attack on the wing. Low pressure also increases with increased airspeed. If we are going fast generate high angles of attack, we pull lots of Gs, hence the reason vertices are commonly seen on maneuvering aircraft instead of aircraft flying strained level. In my experience, I have never created vortex trails under 4 Gs...I will be shocked if I ever have the opportunity to make them in a WWI fighter.