For what it's worth...

I've had no end of trouble with a 2820 running firmware 3.3.7.5 when it comes to IPSEC and PPTP VPNs: Damn thing would barely keep the VPNs (x2) running for 30 seconds, and they wouldn't pass any data. I tried everything I could think of and must have re-setup the VPN definitions 5-6 times at least. Things started to work fine again, straight away, when I downgraded the firmware to 3.3.5.2. The other ends of the VPNs were a pair of 2800s with latest/last firmware.

The only 'benefit' I can see of attempting to work with 3.3.7.5 is that it includes a fix for the OpenSSL vuln. Maybe Draytek will have a stab at getting the firmware fixed, but I feel their focus has moved on to newer models.

I have an IPsec VPN running on 3.3.7.5 with no problems.