Is it possible to extend the 15 pin cable from the VSM-300 XT to the I/O box by using a standard DB-15 computer cable?
Yes, it is possible to use a DB-15 standard cable. However, the TC-Helicon cable is different and connected as shown on the attached PDF.
The difference between the computer cable and the cable with the VSM-300 XT is that the computer cable will not have twisted pairs as shown in the spec. This will result in greater cross talk between channels - ie. if you have a high output keyboard on the line input and a dynamic mic in the mic input, if you turn the mic level to really high and turn off the line input level, you will hear a small amount of the keyboard in the mic channel when using the computer style cable.
This is still a very small amount and only when the mic level is set very high.