Edit vars appropriately.
What is vars, and what is “appropriate”?
./easyrsa build-ca
I do this, but terminal asks for ca passphrase. Where does this found? i.e. is it my VPN passphrase, or the ca key? Or something else?
EasyRSA configuration variables.
Appropriate to your configuration, although it is not mandatory.
In general case, you need to specify a unique passphrase for each private key including the CA private key, however you can avoid it with the nopass flag.