Diane,
I can provide you with this that I wrote for my site, which will display an error message at the top of the ADD PRODUCT form if the vendor has not entered their Stripe payment method. It does not allow them to add a product unless they have connected to Stripe.
This gist shows your product-edit.php template file with the code added; you will need to edit that template. See this if you have not already edited templates (I think you have):https://www.wcvendors.com/kb/changing-vendor-templates/

https://gist.github.com/fervous/29305b1e2eae55dec22eaec32bb1adef

1. You will need to add LINES 24-28 that you can see on the gist, in the precise location that you see in the gist:

/**
 *  Ok, You can edit the template below but be careful!
*/
$access_token = get_user_meta( get_current_user_id(), '_stripe_connect_access_key', true );
if ( !empty( $access_token ) ) {
?>

2. Then, You need to scroll down to the bottom of the gist and look at LINES 210-215 after the </form> Again, you need to add the code in those lines, and you can change the message to display whatever you wish and since I wrote this for my own site, you will need to change the link to lead to the proper address for the pro vendor dashboard page on your website.:

<?php
} else {
echo '  <strong>WAIT!</strong>   Before you can add products, you must connect to STRIPE, so you can receive the funds from your sales.  To connect to Stripe, <a href="/shop-dashboard/settings/"><strong>VISIT YOUR SHOP DASHBOARD SETTINGS</strong></a> and click the <strong>PAYMENTS</strong> tab.';
}
?>

Remember that when we update the product-edit.php, you will need to update your child theme version of the template and place this code in there again. We do notify everyone of any template changes when we release an update in the changelog.