The English club need to beat their Spanish visitors or play out a goalless draw to progress to the last 16, otherwise Valencia will go through to the knockout stage at their expense.
"We just have ourselves to blame," Villas-Boas, whose team have three times given away points when they had led the game in this European campaign, told a news conference on Monday.
"During this campaign of five games we have done pretty well in terms of performances but the small details have run away from us in the last few minutes... and if that had not happened it would have been different."
Valencia are one of the teams to have profited from Chelsea's carelessness, converting an 87th-minute penalty to snatch a 1-1 draw at the Mestalla in September.
Bayer Leverkusen grabbed a 90th-minute winner in a 2-1 victory last month that put the German side through to the next stage and the Premier League team also let Racing Genk equalise in Belgium.
Villas-Boas said his team would be thinking only of a win at Stamford Bridge.
"People have seen us play enough now to be sure we always take the initiative in games," he said.
"We know that Valencia want to get something because a goal for them makes the game even more difficult for us, but we won't change our strategy just because it is an important game."
The Portuguese manager's future has been the subject of much media speculation with his side having lost three of their last four home matches and it is only likely to pick up pace if Chelsea fail to book a ninth successive last-16 spot.
"We cannot speak about going out. Tomorrow we just have to focus on the game," Villas-Boas said. "I have not thought about it and what it would mean to the club. Our focus is on tomorrow's game."
He urged the Stamford Bridge crowd, who he says have been noticeably anxious of late, to recreate the atmosphere of previous big European occasions, such as the 2005 last 16 second leg victory over Barcelona, to help out the players.
"We face Valencia in one of their most impressive moments of the season, different from when we faced them last time," he said. "Our fans can have a massive impact and I have good memories of top European nights in this club.
"I know the atmosphere can be raised and if the fans can replay the type of atmosphere they had in the 4-2 win over Barcelona then we have the right ambience to take on Valencia."
