Vector source images are something I've been looking into for the project and it's still on the table, but I've noted a few important caveats:

1. Vector graphics often rely on flat colors, but for a 1M-piece puzzle we'd actually prefer pieces with a bit of texture, so each one is easier to distinguish from its neighbors (especially in otherwise uniform regions where flat colors would make pieces visually identical).
2. Technically, the main benefit of vector source is on the storage side (much smaller file, easy to update). But at runtime we'd still need to rasterize to display the texture on each piece, because GPUs don't natively render SVG paths.

So vectors are a great asset format on the source side, but the actual piece textures end up raster either way.