CS-Cart API allows to populate with the products from this vendor.
API — CS-Cart 4.2.x documentation
As for another feature (the backend system will automatically place the order with the vendor), additional code modifications are required to integrated CS-Cart with 3rd party API.
Our team will be glad to help you with this modification. Feel free to contact us if you have any other questions or interested in the customization.
But just to be clear, the cs-cart API is only going to let that foreign site populate using cs-cart API format (which most won't support). So you will still have to have some custom work done to read their data and translate it to cs-cart API format or simply to import it directly like via a provided CSV (which many suppliers will supply and you just need to change the column header names). I.e. you'd have a csv for the initial import and then another for updating the quantities hourly or daily or whatever.
APIs are wonderful. It's just too bad there's no standardization between them without going to something like EDI.
The suppliers functionality can be used to hook into the ordering process for your suppliers for items within a single order. But again, you're going to have to have custom api code written for each supplier since I doubt they utilize the same formats or methods.