To my ears, bass in all manner of ways sounds the same to me, albeit with more speed now.
Mids are also the same.
The differences lie in the tonality and treble, which more than likely go hand in hand.
The vegan pads sounds more neutral, compared to the leather which sounds warmer. The treble is better integrated into the sound, with less congestion, and more air. The soundstage is also improved. It no longer sounds like a wall keeps sound cues from reaching further out.
That all sounds like vegan is better than leather, but the tradeoff is that the LCD2 isn't as warm as I remember, nor as velvety. I mean, it is still warm and rich, but not AS much. It's more tonally accurate.
While I compared the leather LCD2 to my K702 Anniversary, it was no question that the LCD2 was very thick, warm and congested in direct comparison. Much warmer. Now with the vegan pads, it shares a similar warmth and smoothness, that comes closer to true balance on a warm side, than a heavy tip towards warmth. The vegan LCD2 is still an LCD2, but that extra treble energy at 10khz basically fixed the treble roll off, which is a double edged sword to me. It makes the LCD2 technically better overall, but slightly takes away from that chocolatey goodness you get from leather LCD2 pads.
I would almost consider this a Hifiman-infused LCD2.
I wanna state that it's a subtle difference. You can expect warmth, incredible depth and texture in bass, stellar mids. But now, you can also expect a slight bit more agreeable amount of treble as well. If you want super smooth treble, go with leather. If you want a hint of sparkle, vegan is the way to go.
Which do I like more? Hmm, tough. There are times I miss that really smooth, chocolatey LCD2 sound, but there are times that I prefer the clearer, more airy sound. Either way, I'm happy, as bass and mids are amazing on both.