By all means keep the custom openvpn port, I find that practice as reasonable, bots and what not scanning services causes spam, the problem is tho if you get used to seeing that spam, then the one day you have a legit attempt at your security you likely to ignore it as you just used to seeing daily spam. Which is why I use custom ports for non public services a lot of the time.
On the question of things like snort, I wouldnt bother in a situation where the one and only listening service is a private VPN server.