Will my PC with BIOS boot a Vega 64 when it does not boot an RX 480

For 6 months I have had a Sapphire Nitro OC+ 8GB RX 480 running in my EVGA SR2 system without error. Note: this is a BIOS motherboard.


During gaming my PC suddenly dropped power. While temperatures of my card in Overwatch normally showed 34~45, I was not able to hold the card in my hands for over a second.
I was not able to boot it unless I plugged in a different GPU. The fans on the GPU never started spinning and the BIOS beeped like mad.

I send the card back to the manufacturer who replaced it for a refurbished model.
I cannot get my PC to boot or recognise the refurbished RX 480. The BIOS posts an error code for no GPU being detected, but the refurbished card works perfectly in an AM3+ motherboard.

 

My reseller claims on distance that my mainboard/slots are malfunctioning. Meanwhile a 750TI and a HD7750 work perfectly together in my system.
They offer me 60% of the cards value for refund or sending the refurbished card back for repair...

 

I read many forum posts about the RX480 being incompatible with older motherboards, BIOS motherboards.
Vega has been released and is based on the same infrastructure. Will Vega 64 be compatible with my motherboard? If it works I might go for two cards, but I am not at all prepaired to spend 1K euro to end up with incompatible cards. It leaves me wonder why the RX-480 functioned for 6 months in my system without error or can there be an issue with a refurbished model versus BIOS mainboards?