$myArray[$indexOfObject]->total_venues = 6;
$myArray[$indexOfObject]->total_bookings= 14;
Your example:
foreach($vendors as $key => $vendor)
{
$total_venues = $this->po_model->get_count_venues($vendor->user_id);
$total_bookings = $this->po_model->get_count_bookings($vendor->user_id);
$vendors[$key]->total_venues = $total_venues;
$vendors[$key]->total_bookings = $total_bookings;
}