In order to allow users to add from one vendor only please add this code in the function.php of the theme :

add_action(‘woocommerce_add_to_cart_validation’,’woocommerce_add_to_cart_validation’,10,3);
function woocommerce_add_to_cart_validation($passed, $product_id, $quantity){
foreach (WC()->cart->get_cart() as $cart_key => $cart_item ){
$cart_vendor = get_mvx_product_vendors($cart_item[‘product_id’]);
$product_vendor = get_mvx_product_vendors($product_id);
if($cart_vendor && $product_vendor){
if($cart_vendor->id != $product_vendor->id){
$passed = false;
wc_add_notice( __( ‘Another vendor product is already in your cart.’, ‘woocommerce’ ), ‘error’ );
return $passed;
}
}
}
return $passed;
}