Graphic card or cloud computing

what is more efficient to crack wpa 2?
3 vega 56 cards 200 euro each or cloud computing?

This depends on your use case.

If you have a one-time cracking job that will only take a few hours or days, then the cloud makes more sense.

If you intend to have longer-running jobs, or multiple jobs over time, then as soon as the cloud costs 600 euro, you break even for cash out of pocket (not counting the value of your time to set things up, etc.)

You'll have to do the math for your own situation.
Also avoid buying AMD cards for Hashcat, or really research the subject (benchmarks and "PCIe atomics" especially) if you absolutely want to.

See for example:
"Vega will overheat and throttle very quick"
"The power consumption is half of what killed Vega. Unless you’re water cooled, you’re guaranteed to overheat within a minute or two. Stability is nonexistent in a large chunk of AMD cards."
"Just make sure you control the thermals.. they have the potential to be extremely power hungry and prone to overheating."
"If money isn't an issue and you need that GPU for hashcracking, get an NVidia."
Etc etc.

TLDR: check Hashcat benchmarks for 1, 2 or 3 nVidia cards that you can get for that 600€ budget Smile
thank you for your answers.

I could get two 1080 gtx for 600 euro.

1080 gtx about 400.000 wpa 2 = 800.000 wpa2
vega 56 about 370.000 wpa2 = 1.110.000 wpa2

what if I undervolt the vega?

Should I buy the two 1080 gtx?