Vega is ... hot. In stock it takes about 400-450W in dual, 300-350W in ETH-only mode at about 33MH/s. And it's throttling like hell. It's very good in dual mining mode, but it's too hot.
Have you messed with undervolting?
Usually I don't change anything, just use stock clocks/settings. But for this card I will have to change some Wattman settings due to hard throttling, but I found no Wattman. I spent a few days to prepare my tools and code for Vega, but today I finally noticed it: no Wattman for Vega at all. May be because I use Windows 7 instead of Windows 10, I will check it (though it's strange anyway). Also I don't see "gaming mode" button in drivers. ADL does not work too, so I even cannot set 100% fan speed to prevent throttling. AB does not work too. Weird.