It depends on the protocol you use for the vpn. The more "standard" ones use PPTP and IPSEC. These use protocols that are pretty much only used for vpn so it is obvious. Most VPN services still support those but many now use what is called OPENVPN. This uses the same ports as HTTPS and since HTTPS is extremely common it looks like most other traffic so they can not easily detect it.
Now countries like china have figured out that OPENVPN does not follow the HTTPS standard directly so they can detect it by watching the messages sent when the tunnel is established. They have even found patterns that they think are unique to SSLVPN which actually does follow the HTTPS standard and is used by corporate gateways.
OPENVPN does not perform as well as IPSEC so it is still a better option, most problems with ipsec vpn are not the ISP blocking it but other technical issues getting it setup.
Still a ISP really doesn't care you use a VPN. Even if you were to use it for illegal stuff they can rightly claim there is no way they can tell what you are doing. All the traffic will appear to come from the vpn service not from your ISP.
The only people that seem to care are things like netflix and other video content providers. They collect IP blocks used by VPN providers and refuse to allow you to use their service since they think everyone is trying to get around location restrictions.