VPN Plus is the variant of VPN Server that is only available on SRM routers. But both are VPN servers that terminate connections from other devices. Don't get this confused with creating an outbound [client] connection from DSM/SRM network settings to an Internet VPN service ... often when people talk about these they are imprecise in how they phrase it.
There are two basic modes of VPN service here*:
User-to-LAN, commonly referred to remote access
Site-to-Site, aka LAN-to-LAN
The first type enables a device (client) to create a secure connection back to the VPN server/concentrator. The server then bridges these connection onto a LAN connection. Typically the device will have the same, or similar, access to other LAN devices. DSM and SRM provide various implementations of VPN server for client access:
Free options (and the only ones available in DSM)
PPTP (avoid unless it's the only option you have);
L2TP/IPsec (natively supported by Win/Mac/iOS/Android);
OpenVPN (requires third party client app, but considered the strongest option of the three).
SRM's VPN Plus Client licence service:
SSL-VPN (dead easy to use on mobile devices);
WebVPN (clientless access to LAN 'web' applications via the web portal);
Remote Desktop (clientless access for VNC and RDP by the web portal);
that other Windows-y one.
The SSL-VPN plus WebVPN combination is a lite-style of Pulse Secure (who are the latest incumbent of this, you may have heard of it when owned by Juniper): there is none of the host checker validations, enhanced clientless access, other applications via the portal. Probably more like SSL Explorer was in the mid-2000s.
The Remote Desktop is, so far for me, a bit hit and miss on which VNC servers will work and progress to desktop view from the black screen.
The second type of VPN enables two VPN routers to create a tunnel that permits LAN devices on either side to communicate with each other. The effect is to span the LAN through the tunnel, though the subnets will be different at each site.
*ignoring types such as VRFs used to create virtual networks within MPLS.