Well it is so cheap to assure the connection with a totally secure VLAN, that you may as well go with that. If you decide upon the VPN option then you will save the costs and also can expect up to about 65% of your slowest connection throughput through the VPN for copying which is great. If the system is not used whilst the backup is running, then IKEv2 will be extremely reliable and you will have no problems with performance. If you need the system during backup hours, the backup will steal all of the bandwidth and ruin performance. If you were using less reliable connections, the VPN solution would suffer, as it needs to renegotiate security after every drop in connectivity, but from a data centre, you will be fine.
We recently tested two 3910s over 100MB fibre at each end and the VPN didn't drop once in a month and gave around 70Mb throughput.
