There's a way to make the nights credits to post automatically. You need to make sure that your Marriott Bonvoy member number is attached to the reservation before check in and that the upcoming stay shows up in your Marriott Bonvoy account.
To attach you Marriott Bonvoy number to the reservation, you can call the resort and ask them to attach it. You will then see the upcoming stay appear in your Marriott Bonvoy account right away.
If the resort is unable to attach your Marriott Bonvoy number to the reservation, then you can do it yourself. You will first need to obtain the Marriott reservation number. It is different from Vistana reservation number. You can either call the resort and ask for your Marriott reservation number, or you can find it in an upcoming stay email from Vistana. They send out these emails a few days before check in.
Example email subject: "Your Upcoming Vacation at The Westin Lagunamar Ocean Resort". Towards the bottom you will see a confirmation number. This is a Marriott number that you need. Here's the screenshot example:
Once you have your Marriott reservation confirmation number, the easiest way to attach your Marriott Bonvoy number to it is through the Marriott Bonvoy mobile app. In the app, go to Trips, scroll to the bottom, click Find the Reservation, enter the number, and then add it to your trips. You will then get the nights credited automatically.
Don't wait until check in to attach your Marriott Bonvoy number, do it in advance.