Why couldn't he be in Porre? Do we have reason to believe that Porre would antagonize Viper directly?
Porre is considered a country as I recall... considering that fact, it is likely very large, and as I mentioned, the CT maps and actual landscapes may differ greatly in scale... Unless we are to believe that there really are only a handful of people living on the planet.
Another thing, about the "remote town of Regiorra," this phrase, depending on the original Japanese, could mean that the town is an outlying part of Porre. Or simply put: The town of Regiorra is somewhere on the edge of the civilized portion of the Porre territory. It is 'remote' in the sense that the coast is remote from the center of a continent. We also don't know where Serge is from initially as I recall, so he could be referencing his own origins, meaning, "Compared to my hometown, Regiorra is remote," or "Compared to the capital of Porre, Regiorra is remote."
Because the word 'remote' is so subjective, and can be even moreso in Japanese (which is a very subject-relative language), we can't say exactly what he means here.
By the same logic, if Viper was in Regiorra and that is the same place as Regionna, which is an outlying town of Porre, he would have some manner of buffer zone between himself and the major force of Porre. Military forces tend to be denser toward the capitals and strategic centers of countries, and not necessarily nonexistent, but slightly diminished in the outer "less important" portions of the country.
As I recall, Guardia was (before its fall) the largest territory in the world, yet it only has one small burg to its name. Thus the CT maps may not show us the entire country including every city and house (99% unlikely in fact) but may in fact represent the entire country as a single town; the quintessential Guardian town. Likewise, each group of houses could represent a town...(unlikely as there is only one mayor to each 'town,' and seems to be at least one for each as well).
In CT, Porre was never really important, so it could be that the Porre town we see is Regiorra, it despite the fact it is the only town we really see in Porre, it may still only be an outer town.
Small cities are better hiding places than large ones, due to the relative size of the law enforcement there, to cite real world examples... The US has greatly increasing gang activity in smaller, largely unheard of cities, than in the big cities everyone associates with gang violence. The reason is simple: Small towns aren't equipped to deal with them, and they can run amok.
For this reason, Viper could have chosen Regiorra as his 'in plain sight' hiding place for the Flame.
Two final notes on why he might have a mansion in Porre itself. First off, Vera assembles a strike force rather quickly to attack the manor, meaning that even if they were in an outlying city, there would have to be Porre soldiers/police/military police relatively nearby.
The second note is this: Why do we assume Viper wasn't a respected citizen of Porre? I can't seem to find any information as to why Porre and Viper would be at odds in Radical Dreamers. Perhaps in Chrono Cross, yes, but there is no outright evidence I can recall or find that states they were enemies. If the Acacia Dragoons were in fact a part of Porre to begin with, it would make sense that Viper would have a mansion there.
I also ask, do we know the Flame was 'passed' to him willingly? Perhaps he led an assault on Guardia himself and claimed it as part of his spoils. In the former case, it's not unlikely that as there was no stated tension between Porre and Guardia before The Fall, Viper could have been a loyal friend to Guardia, and thus was entrusted to its safekeeping, even knowing he couldn't prevent the seizure of the kingdom itself. How do you hide something from being discovered by your government when they search for it? You keep it with you, because why would they suspect you of having it when it should be in the vault?
I'm not stating that it is an obvious conclusion, or even a well founded one, but it is a logically plausible theory.
