Modric has spent pre-season training away from his team-mates, but with the Premier League kick-off approaching his boss wants him back committed to the Tottenham cause.
"He is training alone because we decided to do that until his mental state is ready to return to the team," Villas-Boas said. "It is important that in the coming days he understands he must return to the first team. We respect the player's ambition, but he must also respect the club."
Reports in the Spanish media say that, with Paris Saint Germain no longer interested, Tottenham chairman Daniel Levy has lowered the asking price for the midfielder from €45 million to €38 million. ESPN understands, however, that this is not the case and that Spurs have not reduced their demands.
With Madrid sources firm that Jose Mourinho's side will only pay €30 million, the Croatia international could have his intention to leave White Hart Lane thwarted for a second successive summer.
"Until we receive a suitable offer for the player, he will not go," Villas-Boas said. "I do not know if it will happen, but my job is to get his head right to play for us. It is important that in the coming days he realises he must prepare to play for our team.
"The contacts we have had with other clubs have not met with what Tottenham wants for the player. We respect the size of Madrid and the other interested clubs, but they must respect what we are doing too."
Villas-Boas was speaking after a friendly at Valencia's Estadio Mestalla on Thursday night, where Tottenham were beaten 2-0, with Mauricio Pellegrino's side easing to victory courtesy of goals from Brazilian attacker Jonas and French midfielder Sofiane Feghouli.
The Portuguese coach suggested that, had things gone a different way, he - not Pellegrino - could have replaced Unai Emery as Valencia boss this summer.
"Someday I would like to coach in Spain as it is an interesting league," he said. "The first person to call me after leaving Chelsea was the Valencia president (Manuel Llorente) and that was important for me. I did not come here, but not because I did not like the sporting project, or the size of the club. It was more for my feeling that I needed to go back to the Premier League and try to do well there to get back to the winning dynamic I had at Porto."