Vega is good card, mixed rig with 570 is no problem (in one rig i have 3 rx 56 samsung, one hynix, and 570rx armor and zero problems, not restarted for a few months),
works great, no problem at all.
Hashrate problem on vega cards, at least in my case (no mater if it is 56 or 64, even brand dont play a role) is that aluminium fins and copper base are joine together
with solder and that solder in my case melted off atleast 70% of surface (my frend had same error, and he did not belived me until his cards performed poorly and it was the same thing
).
If jou have hashrate problem use mobile phone flashlight and look space between aluminium fins and copper plate, if light comes between, than your heatsink is bad.
No you cant resolder it, i tried few times with heat gun, heatsink exploded hahaha.
On this cards then i mounted cpu wattercooling (DIY) works best, and arctic acelero 3 (DIY), on one card i even used old intel 775 cooler (driled holes to match holes on card, and
what you know it works).
my hashrate on cnr 2150.
So my point, if u buying used blower vega, take a look at heatsink, becuose this is maybe the reason why guy is seling it
And im not definitly alone, look at this guy
https://www.reddit.com/r/Amd/comments/9eqhnc/my_vega_64_le_was_having_horrific_throttling/