RE: What if there are multiple production orders a single order but their output is not exactly known in advance.
You should only pay VAT on the value added by the vendor - you are not selling the issues and buying the item back - you are buying the contracted service and any RM used by the vendor - in effect the vendor is just another step on your route. However without knowing what to order you would have to have e.g. have a monthly Order for 1 of the most popular variant and add a lot of byproducts or co product lines for each variant produced for that month.
You will have no visibility on your wip until complete unless the vendor gives you interim feedback .
You could however do a manual process and then try to automate it e.g. daily movement against the vendor stock based on a daily stock return to write it off form stock, and another to receive finished stock . A data load template could streamline that.
The challenge is that mrp does not see demand or supply until it happens and has to be driven by min max settings and regular stock updates.
You should be able to choose appropriate accounts to see what has been 'expensed into wip' and the new value of stock made, and total cogs but it might be tricky to get it by specific variant, and any material scrap loss might have to be allocated based on number of pieces produced or direct COGS.