Some people do find that changing to a vegan diet does aid in their recovery and reduces soreness. This doesn't happen for everyone, but occasionally I do hear about such a thing happening, which may well be attributed to better post-workout recovery for some after going vegan.
Soreness will also dissipate more over time as your body adapts to the training you're doing, not to mention, varying rep schemes tend to have a strong impact on overall soreness aka DOMS.
When I trained low-rep with near maximal weights, I NEVER felt a "pump" and was rarely sore. I made my best progress this way, as soreness is NOT an indicator of progress gained in training.
When I dropped down to do more moderate to high rep lifting in years past, I found that I was sore more often. When I train 5 reps or less, soreness just isn't a factor for me, but if I'm training well, the gains come quickly even if I feel fine the next day. Now that I train more with sets in th 8-20 rep range at this time, I'm more sore than I was in the past (but find that gains aren't coming as fast as when I train heavy for low reps). Some people do better with higher reps, for me, it primarily builds more muscular endurance and helps with fat loss, and is the only way that I get any real soreness after training.
Could be other factors as well, but perhaps your recovery has improved with better nutrition on a vegan diet, or, you're just adapting to the training at a more rapid pace than in the past, allowing for faster recovery, too.
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous