I own the 2500 and the VCA. I bought the 2500 first years ago. When i first got the 2500, i used it on the mix buss. It has a lot of options for tailoring the compression, but the sound, while great, definitely isn't for everything. Realizing i wasn't completely happy with it, i got a Smart C1 for the mix buss, and i moved the 2500 on to my drum buss.
C1 was nice, but recalls were a bitch, and i didn't always love it's color.
Finally got the Overstayer VCA, and am very very happy with it on the mix buss. It has a more neutral tone than the smart, which for me is better on the mix buss. Also way more options for shaping the compression, and way easier to recall.
Yes, the overstayer is an SSL style. But with a blend knob, an optional high pass in the side chain, more attack and release options, and a very simple but sweet high and low shelf eq. Oh, and a "Behavior" knob that just does the craziest extreme compression.
As soon as i got the VCA, i put it on my mix buss, and it hasn't left.
The API 2500 still lives on my drum buss. I love it there. I'm sure the VCA would be great on the drum buss, and i've entertained the thought of selling the API and putting another VCA on the drum buss and pocketing some dough. But everything just works great now and i don't want to mess with what i have going. I'm lazy.
I'm talking about these completely in the context of mixing. When tracking, i love using the 2500 for gentle compression on just about any overdub. But i mix way more than i track. The Overstayer FET compresser is great for the same thing. In mixing, my fet lives on the lead vocal.