When used as a therapeutic, RNAi ( RNA interference) has the potential to create a highly specific drug with low toxicity. Short interfering RNA (siRNA) is a new evolving technology that is part of a self defense host mechanism. Self regulating and silencing of specific genes happen in nature. Is there any current anti-viral drugs that are based on RNAi or siRNA technology? They must have use them as vaccines against cancer, viral infection, aging?

That's not how vaccines work.

To prevent someone contracting a flu infection using RNAi, you would have to infect ALL of their lung cells (or at least the epithelial cells) and then you would have to hope that the flu virus that the person comes in contact with expresses mRNAs with exactly the same 21-23 nt sequence. Although doubtlessly there will be some more conserved regions, the likelihood is that mutation will render the therapy useless.

If you did know the exact sequence of the viral genome then you'd probably opt to express one or more of the coat proteins and from these, derive a vaccine.

I advocate the latter point. But science need deploitation and innovation. Try boldly...