Hi SensualParty,
I have done exactly this for another company, basically you need to
1. Get the stock info from your vendor using their API , store this stock info into your database
2. Update the stock info back to Amazon - for products who have no stock - using Amazon API.
3. Run both of these programs automatically every 1 hour (can be less also) so they will auto download and update the stock info.
I have done this for windows using .Net. This is assuming you have a existing database of your products with ASINs and already have mapping of your products to the vendors. If not then you will have to build the database and mapping first.
Do let me know if you have more questions.
Thanks,
Ashish.