Diploma

You should opt for engineering after completing class 12th.

For more details, check out the link mentioned below:
https://engineering.shiksha.com/diploma-in-computer-science-engineering-in-delhi-ncr-categorypage-2-55-598-0-0-10223-1-2-0-none-1-0

Hi Devendra,
Diploma in mechanical is a good choice to make your career in mechanical either design or core mechanics. I don't know what makes you want to take tourism. Both the courses are different in nature and career opportunities.If you want to start your career in tourism, then go for it, but think why you opted for tourism course? Think about the options you have at present in mechanical. Compare both the fields and scope they have. Remember degree is not enough to start your career.
Can I know the reason behind your decision?

Your Diploma cannot get you admission in abroad universities, so try to complete your B.Tech, then try for abroad studies.
All the best.

As per the value of a degree, there is no such difference between B.Tech and BE, so you can choose anyone. Private colleges provide B.Tech degree and government colleges provide BE degree. So, you should try for better institutes, like:
1. IITs.
2. IISC.
3. NITs.
4. BITS – Pilani.
5. KIIT, etc.

Yes, you can apply for government jobs after Diploma in computer applications. Some of the vacancies available in different government agencies have criteria of Diploma in computer applications. These are very less but available.

Yes, you can study VFX after your Diploma. As the Courses offered for VFX has a wide curriculum, you will get multiple job opportunities.