The drains in sand filters usually have something to prevent sand from draining. In my Hayward it's a 'short' lateral. When they assembled the filter at the factory they forgot to put in this short lateral. On the first drain for winterizing a lot of sand came out. I just dealt with it. Siphoned when I removed the valve assembly. When I removed the sand to do some other work I made a lateral of the right size PVC pipe and cut fine slits in it. Yeah...could have gotten one from Hayward but this was less of a hassle. Bottom line, trying to wash sand out the drain won't work. Shop vac like you used is the easiest I think.
Al