Wow, that sounds like a great opportunity!
When I wholesale I do not have any agreements. I have a brochure online with pricing charts based on the amount purchased as well as the minimums required for a particular price for the items, and terms such as the purchaser must be a business intending to resell the product, but this is a general information for everyone and the terms that I am selling under.
But generally speaking I do not think you need any agreements between you and the wine place wholesaling your soap because you can have your terms laid out that they agree to by purchasing your product (or else you would not be selling it to them if they did not agree on your terms).
I think that if the winery is asking you to add the wine to your soap that they should be supplying the wine or giving you a huge discount. It is a tricky thing to think about, but I look at it as you would sell a non-wine soap to them for x wholesale price, so if they ask you to add wine that increases your costs to x+wine, thus the cost to them increases as you cannot absorb the cost of the wine, so for them the cheapest way to offset the extra cost to themselves would be to supply the wine for free. If they didn't give you the wine for free then obviously your wholesale price for wine soap would increase due to the costs of the wine. Perhaps you could explain to them that X is the base wholesale price of your soap without wine, and they can pay a higher price if you have to buy the wine or they can supply you with the wine for free as it is for THEIR end product (I mean that in a nice way of course). Of course for them it would be cheaper for them to supply the wine, instead of you having to buy it, put it in the soap, and they buy the wine back as soap. In essence they are not selling wine to you as benefit to you, because they get the soap in the end anyway, so you should not be obligated to give them a discount on soap if they give you a discount on wine, because they get the wine in soap form at the end anyway!
I am sorry for rambling on, just ideas that come to mind. I guess it is important to get the price right before you give them a final offer and then you realise later that you are not making much profit. I have realised from selling that it is really easy to under price and end up working for free (which soap making is fun), but is a bummer when trying to support yourself. Ok, I'll stop now! hehe!