FTP is an insecure protocol, you rightly want to improve on that.
There are several possibilities:
- replacing it with a secured version of FTP, or moving into another transfer protocol such as SSH (the "SFTP" part)
- tunneling the traffic via a VPN
You chose the second solution, it would be good to reconsider the first one to go for a native encryption of the protocol. There may be other considerations which push you towards a VPN, though.
If you use VPN, you have the following scenario:
- the traffic between your client and the VPN gateway is secured
- the traffic between the VPN gateway and your service (FTP in your case) is as secure as the protocol. Again in your case: not secure because FTP.
Depending on a plethora of elements, you may be OK or not with this unsecure part of the transfer. This will depend on your risk analysis, which is unique to your case.
As a side note - the vendor of the VPN does not matter for your question (provided that the implementation is correct)