Why Vaarsuvius will take the deal.

[Kranden]

Because your own soul is a small price to pay to save the souls of your family, the torment V went through knowing he was not able to save some nameless NPC's from death will pale in comparison of the eternal torment V would suffer knowing his family souls would be lost forever.

[[TS] Shadow]

I think that was made blatently obvious in the latest strip.

[Felixaar]

Au contraire. V will take the deal because V refuses to admit that s/he is weak or a failure, or that leaving her mate to study arcane magic was a mistake. V's dialogue shows the s/he cares far more about his/her own arrogance and pride than the wellbeing of his/her children - not once does s/he mention sorrow over the destruction, buy rather repeats how s/he cannot and must not fail again.

[Kaytara]

Au contraire. V will take the deal because V refuses to admit that s/he is weak or a failure, or that leaving her mate to study arcane magic was a mistake. V's dialogue shows the s/he cares far more about his/her own arrogance and pride than the wellbeing of his/her children - not once does s/he mention sorrow over the destruction, buy rather repeats how s/he cannot and must not fail again.

While V's recently developed fear of failure is consistently present here, I think you're counting the hits and ignoring the misses. Vaarsuvius rants about how he cannot fail again, but his completely horrified and outraged reaction to the dragon's plan has nothing to do with how he perceives his abilities. Months earlier, when V talks to Durkon, he emphasizes the part where he failed to save the lives of countless soldiers and how Haley is suffering because V's spells are failing.
By contrast, there have been many, many, many times before when Vaarsuvius' magic has failed to protect himself or his friends (without resulting in deaths), yet he hasn't broken down about that.

That Vaarsuvius is undergoing an existential crisis due to having failed to protect people he cares about and fearing that the same will happen again does not diminish the worth of his family's lives to him, personally.

[Warren Dew]

V will take the deal because V refuses to admit that s/he is weak or a failure

The very request for aid is an admission of weakness. If Vaarsuvius refused to admit weakness, Vaarsuvius would not be talking to the imp at all.

[Lazy Fat Man]

Au contraire. V will take the deal because V refuses to admit that s/he is weak or a failure.

It didn't bother V at all to admit that he/she had lost to the dragon, when she/he thought it was just out to kill him/her. Therefore, I agree with Kaytara.

[the_tick_rules]

V is powerful, morally flexible,believes power can solve anything, and is currently blinded by emotion. She's an ideal candidate.