I would pipe the OWB in a constant circulation loop like usual, with the 100 plate being the point of return. Then pipe the Viessmann through the other side of the 100 plate. Your existing DHW circ would still get the call from the indirect and pull heated water off the 100 plate to heat the DHW, just as it does now. The other 3 heating zones could be pulled from a manifold, just after the DHW tee in the primary loop. DHW first tee with circ, baseboard second tee with circ, then both radiant zones through a third tee with a circ pulling through a mixing valve and into a pair of zone valves- which is likely already the case if everything is currently piped. Hard to say without seeing your current setup how easy it'd be, but sounds relatively simple. Aquastat on the incoming OWB line to keep the Veissmann from firing unless OWB water is below X temperature. No need for the 20 plate for DHW