Don't know about VM2 but in VM3 virtuemart_manufacturer_id property on product is an array (because one product can have multiple manufacturers). So the code would be this:
if (in_array(1, $product->virtuemart_manufacturer_id))
Or, if you always use only 1 manufacturer per product, you can check against first array element:
if ($product->virtuemart_manufacturer_id[0] == 1)