NOTE: All of this assumes you are talking about TCP 111 (vs UDP).
If you are using a VPN service, you are sharing that Public IP with many customers and being NAT'd somewhere along the way. This Public IP is generally not directly routable to you as a user of the service, as such, there isn't much to worry about.
That being said, it's hard to know (without more info/probing) exactly what is running on that open port. In general, ports 1-1023 fall in the Well-Known Ports range, though that's not to say that something else can't be running on this port, but it's likely an RPC (remote procedure call) service like SunRPC.
TL; DR You're probably fine, but if you're still worried about, contact the VPN vendor and ask them exactly what it is and what it's used for.