Hi Juan,
Do you know if VirtueMart provide multi-vendor support? Unfortunately a lot of ecommerce options don't, and this does make it tricky to use multiple drop shippers.
I would get your developer to look into how other ecommerce sites do successfully offer multi-vendor support, and then look at how to incorporate it into your website.
There are lots of forum posts discussing this problem, as it is an issue for a lot of people wanting to use multiple drop shippers.
I read of this solution someone had for Zen-cart users, which may or may not be useful:
"You cannot deal with multiple shippers and multiple ship-from addresses in Zen Cart or any other cart that we know of. Here's the best way - pick a ship-from zip code from somewhere in mid-America and input that into your Configuration->Shipping/Packaging->Postal Code.
On some orders you will make a profit on your drop-shippers ship-from addresses, on others, you will lose. But it will all average out. That is what most drop ship businesses do."
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Plus there is also this one:
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Obviously this is a completely different shopping cart, but there may be some useful ideas here you can use on your site.
I will continue to look into this and let you know if I find anything else.