Once you have bundled the purchase supplies per vendor, so, your purchase suggestions are actually more or less realistic, you could do the reverse date calculation once again to get an exact match of the date you need to move, and then to confirm that with your production and sales.
So, the problem now is you have to make sure that production and sales people accept the consequences of this overall plan before you’re carrying it out.
So, go again, talk to your sales people and see how good it got with the changes you made to the purchase orders.
Ask your production people if the plan looks overall good enough for you to carry out maybe firm plan or plant production order they can do master capacity planning on later on.
But before sending the orders off to the vendors, you should make sure everybody in the house is comfortable with your plan.