One of the rubs against ceramic tile is that it is cold and hard. There are floor heating systems available to make your floor more comfortable to the touch, but in terms of hardness, yes, ceramic is much like hardwood in that it doesn’t have much give. If you want something that has more cushion to it, perhaps you should consider a floating floor.
Sheet vinyl has come a long way since the original patterns and colors. With the new imaging available many of the sheet vinyl patterns look very similar to ceramic tile or hardwood flooring.
We generally recommend that the underlayment be removed because the subfloor may not be adequate to support tile. However, if your existing vinyl floor was installed over an adequate subfloor using a full spread installation method then yes, it can be done if you use the proper installation products.