This seems to be best for your audience. The gentilicio is "Porteño" but, as you say, that doesn't travel well! "Valparaisians" sounds very ungainly to me (with or without an accened "i").

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Note added at 1 hr (2012-01-05 19:08:02 GMT)
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Maybe something like "the people of the port of Valparaíso, hence known in Chile as 'porteños', ..."would work then?

In Spanish it would be "valpino", without an initial capital. And "valpinos" in the plural, of course.

To my mind the standard option in English is the one you mention: Valparaisan. This is the most common form and occurs in what look like respectable published sources. Valparaisian is rather less usual, and the extra i after the s seems awkward and unnecessary to me. And with great respect to Helena I find Valparisian unconvincing; it doesn't have any real authority, it sounds too much like "Parisian" and it distorts the place name (it's not Valpariso but Valparaíso).

However, if you'd really like a Spanish term, I think "valpino" is an interesting option. The standard term, as has been said, is "porteño", but that's ambiguous (also applying to Buenos Aires, for example) and unless Valparaíso has been mentioned already it won't be understood. "Valparaisino" is correct in theory, but no one seems to like it or use it much. However, some people who actually come from Valparaíso claim that "valpino" is an authentic alternative to "porteño". It is close enough to the place name to be identifiable, but unusual enough to be interesting. Worth considering, I'd say.

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Note added at 7 hrs (2012-01-06 01:16:03 GMT)
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"Memoria de un Valpino
Chile,Valparaíso,
Parque Italia, Enero 2008"
http://ronaldvalpo.blogspot.com/2008/01/memoria-de-un-valpin...

An infamous son of Valparaíso was Pinochet; this biographical sketch refers to his "terruño valpino" (Valparaisan homeland)
http://www.cidob.org/es/content/pdf/1964

"Nome abitanti: Porteño/a, Valpino/a"
http://it.wikipedia.org/wiki/Utente:Kazu89/Valparaíso

This blog is on "gentilicios" (demonyms, as you say) with particular reference to Valparaíso. It starts by pointing out the problem with "porteño": it applies to many other places as well, notably Buenos Aires:

"Con motivo de mi visita a la preciosa ciudad de Valparaíso (Chile), me enteré de que sus habitantes se conocen como porteños [...]"

In one of the replies, a woman born and bred in Valparaíso says that her schoolmistress insisted the correct form was "valpino/a":

"Hola, soy nacida y criada en la hermosa ciudad de Valparaíso y cuando estudié mi educación básica, mi maestra, una de esas fantásticas profesoras normalistas de antaño, nos enseñó que el gentilicio para la gente nacida en nuestra ciudad era valpino-valpina"
http://jamillan.com/lengua/2009/04/26/portenos-y-otros-genti...


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Note added at 7 hrs (2012-01-06 01:23:51 GMT)
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I may say that Spanish "gentilicios"/demonyms are quite often unpredictable, not to say downright weird, and locals tend to be endearingly proud of them. A form like "valpino", rather than the more obvious "valparaisino", is in the spirit of this phenomenon.