We can't say that 2 rx 570 will be more profitable than RX vega 64 it will still depends if how much you bought the graphics card.. Look at the price right now of rx 570 its getting increased and i think if you can buy RX Vega 64 in low price you can make more profit with this card honestly base on what i experience RX vega 64 is more higher hashrate in cryptonight unless if you bought it for low price but if you can buy cheaper rx 570 i think around $300-$330 its really cheap and i will say its more profitable but mostly the price is above $400 any brand..
If you really wanted to mine why not Go to Nvidia instead and mine zcash or ZCL this more profitable coin as of now for nvidia cards and nearly come of ZCL fork..
Edit: adding this just to calculate your earnings and when you break the ROI here check this just put price of your hardware and it will calculate how many months you can break your ROI.
http://www.mycryptobuddy.com/EthereumMiningCalculator