I paid for a 79.99$ game with a 25$ Vanilla Prepaid Mastercard?

With it being a preorder and you using credit cards as a means of payment it will not actually charge the payment option(s) used till approximately 10 days prior to the game being released i do not believe there is an option to do a split payment with multiple
credit cards like that only if between Microsoft account currency(gift cards) and a credit card to make sure what payment option was used for the preorder you may want to go to commerce.microsoft.com and sign in with your account and it will show your order
history (even preorders) and it will show you what payment option was used for the preorder.

If it only used one of the prepaid mastercards when it comes time to charge for the game it will try and take the whole amount off of the one card you tried to use and it would then fail and probably cancel your order.

The best thing for you to do would be to go to the commerce.microsoft.com website and it should allow you to cancel that order. Then if you are wanting to use that 50 dollars in vanilla gift cards i would go to a retail store and purchase a xbox gift card
(not a live subscription card) that would give you the 50 dollars that you can then put onto your Microsoft account and then when going through and making the preorder you can choose the Microsoft account currency as the first payment option and then it would
ask how you want to pay the rest and then you would choose paypal now keep in mind that it still will not charge your account balance or your paypal till approximately 10 days prior to the game being released (game release date is 02/27/2018) so you are going
to want to keep at least the 50 dollars in your Microsoft account currency plus you will want to make sure the money is in your paypal for it to come out.

Makes sense, thanks!

With it being a preorder and you using credit cards as a means of payment it will not actually charge the payment option(s) used till approximately 10 days prior to the game being released i do not believe there is an option to do a split payment with multiple
credit cards like that only if between Microsoft account currency(gift cards) and a credit card to make sure what payment option was used for the preorder you may want to go to commerce.microsoft.com and sign in with your account and it will show your order
history (even preorders) and it will show you what payment option was used for the preorder.

If it only used one of the prepaid mastercards when it comes time to charge for the game it will try and take the whole amount off of the one card you tried to use and it would then fail and probably cancel your order.

The best thing for you to do would be to go to the commerce.microsoft.com website and it should allow you to cancel that order. Then if you are wanting to use that 50 dollars in vanilla gift cards i would go to a retail store and purchase a xbox gift card
(not a live subscription card) that would give you the 50 dollars that you can then put onto your Microsoft account and then when going through and making the preorder you can choose the Microsoft account currency as the first payment option and then it would
ask how you want to pay the rest and then you would choose paypal now keep in mind that it still will not charge your account balance or your paypal till approximately 10 days prior to the game being released (game release date is 02/27/2018) so you are going
to want to keep at least the 50 dollars in your Microsoft account currency plus you will want to make sure the money is in your paypal for it to come out.

I just thought of another way you can do this instead of cancelling the order at the commerce site you can go ahead with the 50 dollars on the vanilla gift cards get the xbox gift cards put them onto your account and then go back to the commerce site and
there should be an option to change how you want to pay for the order.

Or if you want the money taken out right away so you dont have to worry about remembering to have the money in the account you can go ahead and use the vanilla gift card money to purchase the xbox gift cards and then use the money that you were going to
use from paypal get enough of a xbox giftcard to cover the rest and put all of it on your Microsoft account currency and when the whole purchase is made with Microsoft account currency it will take the money off of it immediately instead of waiting for approximately
10 days prior to the game being released.

You should of bought those pre paid Xbox live cards, buying those Vanilla mastercards aren't guaranteed to work 100%.

But I can use Microsoft points to buy a new gold subscription right?

Pre-paid Debit Cards typically do not work for recurring subscriptions, such as Xbox LIVE. You might be able to purchase MSP, but many such cards have restrictions.