Here's why I don't think Sebastian and V. are in fact the same physical person. When Sebastian's mother dies, she bequeaths a small income to Sebastian. Likewise, when Sebastian dies, he passes on to V. that same small income. V. mentions something about this inheritance in the novel's first few chapters, but V.'s earlier (i.e., before the inheritance) dire cash situation doesn't come clear until the end of the novel. So whatever else Don's "pseudo-etymology" may point to, I don't think it means that V. and Sebastian are the same person. Put another way, the inheritance means (at least to this reader) that V. and Sebastian are the same person in the same way that Sebastian and his mother are the same person -- psychologically similar, emotionally similar, motivationally similar, etc.