Suppose I tether mobile net to PC, use VPN in PC, would my dns request go to mobile ISP as well? or only route through VPN?
It depends on the VPN used, I think! The ISP will at least know that you're connected to a VPN Provider, but the real target. Personally, I would use only a VPN Provider which has his own Apps, which you need to install on your machine and which use encryption if sensitive data is used for to transfer. For normal transfers, direct tunneling is good and secure enough, from your machine to the target. Still, the ISP would be able to capture he trafficked packages. If you're not in China, I wouldn't be paranoid!