I have a pipe 11.25" dia and a 2.125" pipe that joins it at a tangent.

Is there a way (in ViaCad 3D v8) to transfer the shape of the smaller pipe onto the larger so it can be printed and cut out to make a template to cut out the hole in the real world.
Using subtract solid tool gives the shape in the lager tube but the tube needs " opening out" to give the correct shape to print.

In ViaCAD, model the junction by adding solid cylinders, then change them to surfaces, surfaces to meshes, then meshes to faces. The faces that surround the junction edge are thickened, all of the thickened solids that make up one tube selected except for one, then they're successively rotated by three points to align the normal edges of the solids, each time dropping the solid which has just been aligned. It's not particularly involved, and compared to the way we had to develop patterns with drawing instruments back in the Stone Age, it's really just process work. I change the normal deviation parameter in changing the surface to mesh from default 7 to 30 to reduce the number of facets, which seems to give a facet count of 40 - 55 circumferentially, and then only thicken and align those facets that surround the junction, which reduces the number to less than half, so it takes, say, 25 iterations to produce a pattern.