It is possibly the latency that causes the performance issues. The VPN may to work too well so keeps disconnecting, and then re-establishing the connection - I have seen it many tomes before. Some, VPN systems will allow for a parameter change to improve operation across the link - your IT department should be the first port of call.
His problem is likely that it's an L2TP VPN which will not work with the TCP acceleration the satcom router does, so TCP traffic will never get out of TCP slow start and will pootle along at ~300kbps max.
Solution would be an SSL vpn (e.g. Cisco Anyconnect).
As for the rest of the query, you could use a Draytek router (e.g. 2830, 2860, 2920, 2925) and connect the ADSL to Wan1 and vSat to Wan2.
Then set a load balance policy to send traffic to your VPN endpoint (whatever server at work you use) over the ADSL link, and either let it load balance the rest, or send everything else down the vsat.
You might find while on the VPN that the machine you are using for the VPN has to disconnect from VPN to use the vsat connection still, if your work have not enabled split tunnelling.