IP pool for vpn-which RFC?

Hi,
I know that for vpn should be used niche IP subnets for example 10.64.234.x…, or we can have ip address conflict.
But, does any rfc define which subnets (from private) should be used to use with vpn?

No, not specifically for VPN. You have to decide how you want to setup your network, and the RFC defines private IP ranges that you can use anyway you want to (not specific for VPN). No matter what range you choose, you have to make sure it isn’t already in use on your network.