I connect to a VPN at somedomain.com; if I have "use default gateway" unchecked, will traffic not directed to somedomain.com just go through my normal internet provider, or will it also go through the vpn?
Basically, yes you have it right. The detail is that it's not necessarily traffic destined at your DNS domain name, it's about IP addresses. For example, in my DNS domain, I have IP addresses in 10 or more IP address blocks, so I have to make sure that my VPN server has the appropriate routes for the IP addresses I wish to go over it, even if they are in the same DNS domain as the VPN server.
I like running my laptop and desktop with the default gateway going over the VPN (everything except traffic to the VPN server, of course :-). In this way I can be out at a coffee shop and not worry about someone sniffing my traffic, I know that it's encrypted up to my data center.
I don't find that the added latency of going to the VPN server first and then up to the final destination is bad. In fact, one of my co-workers says that some places he goes are faster over the VPN with the extra hops, because our data center has better connectivity than the cable provider.
But in short, yes, if you disable the "use default gateway" functionality of your VPN, traffic to networks that don't specifically have routes in your VPN configuration will be routed via your ISP and bypass your VPN.