Here's the deal with using two marksheets (NIOS On Demand Physics and CBSE with other subjects) for private engineering colleges like Manipal or VIT Vellore:

Generally Accepted: Most private engineering colleges in India accept improvement marksheets from NIOS On Demand exams. This is a legitimate way to improve your score in a specific subject.

Transparency is Key: Be upfront about using both marksheets during your application process. Clearly mention that you took Physics through NIOS On Demand to improve your score.

Meeting Minimum Eligibility: Ensure your combined marks (including the improved Physics score from NIOS) meet the minimum eligibility criteria set by the college.

College Discretion: While most colleges accept improvement, there might be a  few with specific policies against it. It's best to check the admission brochure or contact the college directly to confirm their policy on improvement exams.

Here's what you can do:

• Check College Websites: Most colleges have detailed admission information on their websites, including their policy on improvement exams.
• Contact Admissions Office: If the information is unclear, don't hesitate to contact the admissions office of Manipal and VIT for clarification. They can provide the most up-to-date information on their specific requirements.

In conclusion, using both marksheets with clear communication is generally accepted. However, it's always advisable to confirm the specific college's policy to avoid any surprises during the application process.