So, under the firewall rules, lan, I created a rule for every single static IP mapped address, and selected which gateway I wanted it to exit through. So, if I wanted 192.168.1.2 to exit via my isp, I selected that. Seems to be working perfectly. Specific devices are going through my vpn provider, the rest are going through my ISP.
You can make your life a little easier by creating an alias (Firewall->Aliases). Make 1 rule sending that alias to the VPN. Put all the devices that should go over the VPN into that alias. Less "almost duplicated" rules to manage, and 1 easy place to add/remove devices from the list.
You could also allocate static mapping IPs for the "over the VPN" devices to a nice "power of 2" range of your subnet - e.g. 192.168.1.16 to 31 - that means they are all in network 192.168.1.16/28 - then you can make your alias just be the network 192.168.1.16/28. Static mapping something inside that network will result in its traffic going over the VPN. If you are comfortable with thinking/using variable-sized network masks then that can be easy.