Seestar writes coordinates into each Fit and Vespera does not. I was told that Siril latest edition allows stacking by plate solving each frame so maybe if you find and use that option you may be able to stack mosaic. I never tried and probably will not do it. Reasons:
1) platesolving each image will be very slow and super slow on my old mac
2) I will want to keep all the Fits forever and that will require additional storage:-)
3) My Vespera 2 has only 10G of space. I have 5 multi-night projects. I like to run the scope overnight. I am not sure there will be space on the scope to store Fits.
4) People who managed to stack manually did not see much improvement over the Tif done by the scope….
When I was experimenting, I used SETI Astro suite as it has a "Batch" blind plate solve feature.
So I initially sorted all through my Fit files using Blink, deleting any bad ones and then I blind solved all the remaining FIT files, I think after that I tried using Pixinsight Staralignment (Using the Singularity generated TIF mosaic as the reference image) and then Fast integration.
From memory I think it did work, but because there was no linear fit applied to the images it was pretty obvious where each tile in the mosaic was.
As I said I should have picked one FIT image and then linear fitted all the others to get the same intensity etc for each FIT.
I think by this point it was a lot of effort and I lost interest 
