Hi,
Vectrosity would be too limiting i would say, though not impossible for basic line and fill shapes drawing.
Maybe this would be a better solution:
https://www.assetstore.unity3d.com/en/#!/content/19803but coding is likely mandatory, at least to create custom actions to interface with a painting framework.
But the basics is as follow:
Draw pixels onto a texture
Save texture on the device or online as a byte array or png
Use that texture or bytearray in a mesh with the appropriate uv.
Drawing pixels requires knowing the pixel position to write based on the touch screen position, that's key to begin with drawing.
Bye,
Jean