Well, there are a few things that you could do here, but I will highlight one specific method.
Let's start from the more complicated part: The Flask.
You can use the Metamorphosis option from the spell Binding to trap a creature inside an object (like a flask) such that it will be able to communicate:
The subject assumes gaseous form, except for its head or
face. It is held harmless in a jar or other container, which may be
transparent if you so choose. The creature remains aware of its
surroundings and can speak, but it cannot leave the container, attack,
or use any of its powers or abilities. The binding is permanent. The
subject does not need to breathe, eat, or drink while metamorphosed,
nor does it age.
Now - obviously the Wizard will not want to trap himself inside a jar, so he will probably put something else in there that he can control, and to communicate with that creature he will have to create a Telepathic Bond and make it Permanent.
You forge a telepathic bond among yourself and a number of willing
creatures, each of which must have an Intelligence score of 3 or
higher. Each creature included in the link is linked to all the
others. The creatures can communicate telepathically through the bond
regardless of language. No special power or influence is established
as a result of the bond. Once the bond is formed, it works over any
distance (although not from one plane to another).
Telepathic bond can be made permanent with a permanency spell, though
it only bonds two creatures per casting of permanency.
Now all that remains is a subject which will cooperate, and will not oppose the will of the Wizard. I suggest that you could use Simulacrum to creature a virtual copy of the Wizard.
Simulacrum creates an illusory duplicate of any creature. The
duplicate creature is partially real and formed from ice or snow. It
appears to be the same as the original, but it has only one-half of
the real creature’s levels or Hit Dice (and the appropriate hit
points, feats, skill ranks, and special abilities for a creature of
that level or HD).
At all times the simulacrum remains under your absolute command. No
special telepathic link exists, so command must be exercised in some
There are possibly other ways to obtain the same sort of effect, but this one essentially places a double into the flask instead of projecting his image. The Wizard communicates with the double telepathically - and since the trapped double is playing along - can convince the party that it is indeed a real Wizard trapped in a flask. Of course, this method also really does trap someone in a flask, so it's not really a lie. :)