French Schengen visa with valid German Schengen already stamped on passport

[vinzzz]

Hi All,

So here's my situation:

1. I have planned a vacation in Europe starting 25th September with my wife. Our main destination is France.
2. I already have a valid German Schengen visa on my passport (business visa), which is expiring on the 20th of September. This visa was stamped in Chennai.
3. I am travelling to the US next week, and would be returning only by mid-September.
4. We live in Bangalore.

So I went to VFS Global's office today in Bangalore to apply for our French Schengen visas. All documentation was in place, but they told me that first I need to get the German Schengen visa cancelled, and then only will they accept my application for a new French Schengen. They also said that the German visa would need to be cancelled by the German consulate in Chennai.

I then went to the German consulate in Bangalore, but they didn't even let me in without an appointment. Sent me away with a landline number, which operates only Monday to Thursday.

I looked up online but couldn't find any information on how to get a Schengen visa cancelled (I'm assuming not many people need to do it!).

Now my questions are:

1. Is it really necessary to get the German visa cancelled?
2. Can it be done only in Chennai, or in Bangalore also?
3. What is the process, and how much time will it take?

Thanks in advance for your replies and help.

Regards.