Not exactly my expertise - Santana or Dr. Feigenbraun might have more to say here, or disagree. But I'd say yes, it's possible to get bigger and stronger as a vegan, but it's almost certainly not giving you optimal conditions for growth and strength gains. So the gains will be harder to come by, less, and slower than they could be under other dietary conditions.
As far as your lifter, were those starting numbers his actual working weights or did you start artificially light? Bench and press are almost never the same, and I've coached hundreds of men at the far left side of the genetic bell curve, and only one of them in their 20s could only start with the empty bar (excepting obese people). So he would have to be a real outlier. His finishing numbers are weird too - it's odd for press to stay so close to bench, and I'm surprised neither his squat nor DL got any heavier. Even a very genetically untalented lifter, in his late 20s, should be able to get more than that. Though that may be a function of his veganism, so it's all mushed in together.