1) No, vMA is not needed as your definitive syslog, it's just something that VMware provides out of the box with vMA. You can run any type of syslog server or use an existing one. Though I agree that you definitely want to set one up whether that is uing vMA or your own syslog server
2) DRS has no impact on this, syslog is sent from an ESX(i) host to your syslog server.
Note, that ESXi only supports a single syslog server, so if your syslog server goes down, you won't be receiving the remote logs but they'll still persist locally until a reboot.