I have some compact scopes. The Vector Veyron 10x I have is a fine scope, nice and clear and tracks well. For the price it's a steal, but only a 5 year warranty, so if it takes a dump down the road, it's trash. I also have a 3-12X Veyron that has been good, but took a dump the other day. I'm trying now to get in touch with Vector to see if I can get it warrantied.
I also have an Airmax compact 3-12X SF and it is a fantastic scope, clear as a bell and very nice lighted reticle. I've had it on several airguns and it performs every time. Someone said the Vector is better... I dunno about that. I will say, it's not far off, but you're comparing a $450 scope to a $150 scope. Money doesn't always mean better, but I would say the Airmax is a bit clearer, has a better illuminated reticle, better lens coatings, etc. The only place the Veyron beats it is in eye relief, the Airmax has only 3", while the Vector has more, like 3-1/2", it's noticeable when mounting, but it's not a big deal.
Never had a chance to use a Discovery, but would assume them to be in the Vector category, very good.