If you want to hide the fact that you are using Tor, you should use pluggable transports with a bridge. These are a variety of protocols that obfuscate the traffic between you and a supported bridge relay. It functions to hide the protocol and thus the existence of the Tor connection in order to evade ISP-level censorship. The current best obfuscation protocol to use is obfs4. Many bridges support this protocol.
You can visit the Tor's BridgeDB to obtain bridges using pluggable transports for obfuscation.
will my ISP have to try harder in order to discover that I am using Tor?
No, it is easy for an ISP to know that you are using Tor through a VPN. Tor sends traffic in "bursts" of 514 bytes (previously 512), called cells. If the data being sent does not divide evenly by 514, then it will be padded. A VPN, on the other hand, sends data encrypted but without any padding. This makes the usage of Tor through a VPN apparent to anyone who is monitoring packet sizes and can detect those cells. Anyone who sees a VPN send traffic in bursts of 514 knows that Tor is running underneath.
I am in a 14-eyes country, but my VPN is not.
That's not necessarily a good thing. If your connection to your VPN passes the border of an SSEUR country, then traffic will almost certainly be logged. If your VPN is within your country, then the chances that high-resolution logs will be kept is significantly lower, since the small ISPs between you and a local VPN are less likely to have the resources to wiretap. Using an offshore VPN is especially problematic, since your connection to it will be going over an undersea fiber optic cable which is trivial to tap.