could somebody explain to me how drywall is fastened to the inside of an SIP wall. Also, does an SIP house with an SIP roof need to be a vaulted ceiling or can you build a normal ceiling.

You can screw it to the OSB in a similar pattern as you normally would, you just don't have to worry about missing a stud. You can have a vaulted ceiling or a flat ceiling, your choice. SIP's generally don't limit you; they just give you more options.

Steve
GrandCountySIPs.com