Cahill, 26, was in the crowd for Chelsea's 1-0 win over Sunderland after completing his medical earlier in the day.
The England international is expected to be officially unveiled as a Chelsea player in the next 24 hours, and Villas-Boas believes Cahill has all the qualities needed to improve his squad.
"He adds to a very solid back four that we have," Villas-Boas told Sky Sports News.
"Gary is a player of tremendous technical qualities for a central defender, which is what we like because of our possession when we build up from the back.
"He has good stamina and a good sense of anticipation, which were the characteristics we were looking for and we managed to get an England international."
Cahill made 147 appearances in five years at Bolton, scoring 15 goals.