Clogged downspout pipe
If you used flexible hose only for the shopvac (ie no rigid straight sections) you might be able to pick up more debris inside and beyond the elbow in the iron pipe. Suction tube such as that used in pools might be useful, and replacement shopvac hoses (possibly longer than the original) may be available for your vacuum. It might even be cost effective to cut off the rigid coupler from one end of your existing hose so that it can go through the iron elbow further (and buy a new hose to replace the &quot;customized&quot; original hose).
A hole two feet deep isn't an awful thing to dig. If this drain pipe is not covered by concrete or other unremoveable surface you could dig down to the elbow. It should then be relatively easy to remove the elbow, clear the debris, and vacuum into the horizontal section beyond the elbow to remove more debris.