I would doubt very much the slow down in progress is diet related, glycogen is what powers high intensity muscular contractions, and there is an unlimited supply of that on a vegan diet. That being said, are you consuming fewer calories?
I would say your slow down in progress is due to overtraining, yes overtraining:)
Here's something the 99.9% of all trainees and trainers fail to realize, or just plain ignore....they allow for localized recovery, but do not allow for full systemic recovery (which takes considerable longer). Whether its an intense set of bench press, rows, squats, leg presses, bicep curls etc, it all effects the entire physical system. Moreover, As the trainee grow's bigger and stronger, their energy output also increases proportionality, and the greater the energy output - the longer it takes these energy reserves to replenish.
So what am saying is, a fixed frequency program is doomed from the get go, it does not take into consideration the simple biological law mentioned above, that as you train progressively more intensely (which is the only way your body will produce new muscle), there must be a corresponding adjustment made to the frequency and/or volume of your training.
So were training 3 times per week may have been working well for you, it now may be too much, you've become stronger, therefore your energy output is greater and you now require a longer recovery period between workouts, so rather than performing your 3 workouts over a 7 day period, i should perform the 3 workouts over a 9-10 day period, this should allow for progress to resume.
Hope this helps