Does the iron plate inside the piano have to be vacuum cast to be good?

These two are completely different two concepts, there is no great connection. Vacuum casting is a process that uses a vented casting mold. The molten metal flows into the mold by air pressure, and then the air is removed, creating a vacuum. This casting method is mainly used for small parts with delicate details or jewelry, vacuum casting iron plate is just a smooth surface, but the load-bearing is not as good as sanding cast iron, and vacuum casting mechanization is high, low cost, save time and effort, so Yamaha and Steinway these piano's highest end of the big triangle are sanding process.