If you've spent 170EUR on a RPi4, USB adapter and vlan switch then you've done it wrong.
As for VPN performance it's only an issue if you're using something like OpenVPN. My RPi4 easily does 350mb/sec (my current full line speed) to my VPS with Wireguard and I'd suspect it'd do a lot more if needed. May test it out at some point in my LAN.
Sure, that is a solution with lots of reserves, but also with an estimated price-point ~200EUR. I am rather on the lookout for a solution that achieves bi-directional traffic shaping @Gbps rates in the 50-100EUR range. I am not so much looking for my self, my 100/40 link is well within capability of my turris omnia, and not too far out of what my trusty old wndr3700v2 can shape (with not too far out meaning up to a factor of 2 
) but more for a solution to recommend all those users that realize that their old OpenWrt router does not allow traffic shaping at their new rates, so the cheaper the better, and the Nano Pi from above with its $22 price would be just perfect (and for links in the ~500/500 Mbps range it probably is).
In the forums for Espressobin it is made clear to an information requester that although the SoC is listed as having 2.5Mbit RJ-45 connections that that speed is for the SATA connection and the RJ-45s are only running at max 1 Gbit.
It is possible to have this Marvell chip connect at 2.5Gbit but this example does not.
HTH