Vortex can make a very nice scope. They don't make an off the shelf option that is very suited for silhouette. READ: that doesn't mean that a Vortex scope can't be used for silhouette successfully.
The reason why I say they aren't very suited for silhouette is based on a few things.
Reticle. Usually fine crosshairs and a large dot (around 1/2MOA) is what silhouette shooters prefer. Vortex scopes that have at least 24X (assuming that's the area you're wanting to upgrade to) have their EBR, ECR, SCR, XLR, and reticles VMR save for the Viper HS and Crossfire II. The latter scopes offer a standard V Plex reticle and their Dead-Hold BDC. The EBR, ECR, SCR, XLR, and VMR reticles are very busy and distracting as they are including hash marks for ranging, holdover, and windage corrections which aren't exactly easily utilized (if at all) for an offhand silhouette shooter.
Weight. Their lightest scope with at least 24X magnification is 21.4oz. That's a bit on the heavy side. It's not the end of the world assuming you accounted for it, but if your rifle is close to the max weight, you'll go over switching to this over the Weaver T15. Keep in mind that the Viper HS and Crossfire II scopes are 30mm rings which have a tendency to be heavier as well.
Assuming that we're sticking with going with a Vortex scope and based on available reticles, the Viper HS scope is both their lightest scope and one with a decent reticle (along with 1/4MOA adjustments) however its parallax setting is 50 yards to infinity which means its minimum distance is 45 meters. Chickens are shot at 40 meters. So the Viper HS is out. That leaves the Crossfire II 6-24X50 AO at 23.6oz which can focus down to at least 10 yards. Plenty for silhouette.
The Crossfire II is made in
China. I'm not trying to make a blanket statement about Chinese scopes not being up to snuff, but there is a reason why the least expensive scopes are made there. Silhouette is hard on scopes. Most shooters zero in a rifle and rarely make adjustments after the fact. Silhouette shooters twist turrets in multiple directions multiple times a match. Turret quality and repeatability matter here and Chinese scopes make me nervous long term.
The only options that are silhouette optimum off the shelf and currently in production are the Leupold FX3 Silhouette scopes. There is a reasonable price tag associated to that scope though (I would look into contacting Pete for a quote). You didn't specify budget so I'll try and cover a lower priced item.
There will absolutely be compromise with any other new scope in production from the FX3 . Scopes can be made to be perfect for silhouette, but not without modification that will add price, wait time, and potential warranty voiding. Just food for thought. None of that may matter to you.
The closest scope I've found that is reliable is the Weaver V-24 with the Varminter reticle at 17.6oz and focuses down to 50 feet. It's a Plex reticle with a reasonably large dot in the center. Not perfect, but it's close. Decent glass and reliable adjustments. It has about 5/8 of the adjustment range of the FX3 though (40inches versus 27.23inches). You'll almost certainly be shimming the scope rings or using the Burris Signature Zee rings with the offset inserts to cover all of the animals.
Varminter reticle:
http://www.weaveroptics.com/images/reti ... minter.png
Weaver scope with Varminter reticle ($350 shipped):
https://www.amazon.com/Weaver-6-24X42-R ... B0000V2EBC
You could have the reticle changed at a place like
http://targetshooteroptics.com/Services_and_Prices.html , but that is an added cost of at least $110 not counting shipping assuming you have a scope they are willing to work on.
What I'm trying to say is I wouldn't buy a Vortex scope specifically for silhouette if I had the choice. No I've never used one, but everything I've listed here does not need experience. I am not speaking to the clarity of their glass or how bright they are. I would need experience for that. I'm saying they're generally heavy (may not matter to you), have significantly less than optimum reticles, and the one that is the closest to being suited for silhouette is Chinese. No you don't have to the "best for silhouette" scope to be used for silhouette, but if you're looking to buy a new scope I would at least try to land somewhere close. Sightron Silhouettes, Premier reticle Leupold, fixed up Bushnells and Weavers all show up in the marketplaces across the internet from time to time if you are looking to do used otherwise I would buy the Leupold FX3 first and the varminter reticle Weaver V24 second.