You can’t buy directly in vexl, vexl is not a wallet does not touch your bitcoin or your fiat. Vexl is more of a tinder for bitcoin p2p no kyc matching. What you and the counterparty do it entirely on your terms. Vexl has no knowledge of it.
I’m not sure you dug deep into what Vexl actually is. Check this out: https://blog.vexl.it/how-to-do-peer-to-peer-trading-on-vexl-6745f3954ae9
It’s a web of trust model that is based purely on common contacts but does so in a completely anonymous way.
https://blog.vexl.it/understanding-vexl-security-privacy-and-building-a-web-of-trust-867710d4a6fe