sir i only know 20% of +2 if i work hard now. can i get any least rank college in entrance exam? (jee or vit)

No.... But nothing is impossible....U can try day n night and succeed

You need to work hard to crack JEE or VIT exam if your only 20% syllabus is complete.

Focus on important chapters first which carry more marks in the exam. You can check below article to know about important topics-

http://www.engineering.careers360.com/articles/crack-jee-main-2015-focus-more-on-important-topics

JEE and VIT checks student on the basis of concept, speed, accuracy, time management and decision making. So, make sure you practice for complete mock test series for both the exams to judge your performance level and you can improve for your exam.