OK this is the response I said I'd make. I've done a bit more research on this mechanic, and I might have to take back what I said about what I looked at being "conclusive". Based on what I've found, there doesn't seem to be a definite answer to this question -- it might be different, but it's hard to say when it's not entirely clear what the mechanic actually does to begin with. One thing is for sure though, the answer you read is wrong: Vital Spirit, Pressure and Hustle do not increase the existing chance by 1.5x as it suggests. As above, sources seem to conflict on what it does precisely, but what I know for sure is that it does one of two things:
- increases the existing chance by 50%, so a regular 20% would become a 70% chance, or
- directly changes the chance to 50% with no addition or multiplication involved, so 20% would become 50%.
Sources also conflict on whether this mechanic has changed, but the ones that do suggest that that change would have happened in Gen 5. So I'm confident that for Gen 4, the options are restricted as above. I'm also confident that the first possibility is the one that is in effect, as Bulbapedia and Serebii each agree on this:
...if a Pokémon with Vital Spirit is in the first place in the party (even if fainted), the chance of encountering a higher-leveled wild Pokémon increases by 50%. (source)
Regardless, the chance of finding a high-level Pokemon in the wild is increased a lot. Under the system suggested by the answer you quoted, the Floatzel you encountered would have had a 7.5% chance of appearing (ignoring the chance of a different Pokemon appearing). As you identified, this is wrong. Either one of the possible systems I mentioned above would take the chance closer to what you experienced -- 55% or 50% respectively.
If you or I were to test how this works exactly, we could go to an area with where level range is restricted to two levels, and see how often the highest level Pokemon is encountered. Sinnoh Route 201 would do fine as a testing spot -- if we're repeatedly finding level 3 Bidoof and Starly, we know for sure that the first possibility I mentioned is correct, because the existing 50% chance would be elevated to 100%. However if there are any level 2, we know it's the second, since the chance of finding a level 3 would become 50% (ie. no change). Then, do the same in Gen 5 -- Purrloin on Route 2 in B/W is an easy choice -- and see if there is any change.
I can test this myself if you like, but you'll likely have to wait for a response again. :P