https://chainradar.com/xmr/chart
In situations like this I think it is helpful to look at this from a purely economic standpoint: what are the incentives to Bitmain/Baikal/etc. using their own miners before selling them vs. the costs. Since these are foreign companies with near-zero liability to, say, customers in the US or EU, they don't have to worry about individuals claiming they were sent a used or defective product; big farms, perhaps, but not individuals. Conversely, stealth mining the shit out of a high value coin like XMR with your hot new ASIC miner would be all but irresistible so there is a huge incentive to do so.
Frankly, the incentives grossly outweigh the costs, and so you are left with trusting companies like Bitmain and Baikal to act ethically. You can take that bet if you want, but I certainly won't.
The "Vega are huge monero miners" news got widespread in November - which is when Vega supply dried up VERY quickly and the hashrate climb started.
Then the "3'd party" cards hit in Febuary or so, likely feeding the second hashrate surge 'till the price spike collapsed and hashrate went flat.
I doubt Bitmain had a LOT of miners running in that timeframe, though I'm sure they had SOME "engineering sample" type miners running to test out.
Yeah, I definitely remember seeing all the Vega pumping, and I just as distinctly recall them disappearing from Newegg's stock in a matter of days... and ever since then they have only been sporadically available, and at ~2x MSRP (Newegg's price, not some janky reseller's).
No doubt some of the increase in hashrate is from the unprecedented success of compromising servers into XMR mining botnets, but come on, average network hashrate just 3 months ago was around 200 MH/s and now it is 1 GH/s. If half of that jump was from Vegas, and each Vega does 2 kH/s, then AMD must have sold
* - oops, I forgot to divide the 800 MH/s by 2...
