If a vladimir is affected by veigars event horizon or morgana's soul shackle while he is in his sanguine pool,does it prolong the effect and duration of him being untargetable since he is stunned in the current condition?
No the effect is not prolonged.
It states that:
When in Sanguine Pool, Vladimir is still stunned when passing through Veigar's Event Horizon. If he does this, he will remain in his pool for its duration and remain stunned when coming out if the stun has not expired.
The "Stunned" condition does not alter animations or affect any other abilities that are currently playing.
This means that "stunned" characters can still appear to "take" actions that were started before the stun went off.
This means that abilities that make you untargetable are not lengthened by stuns -- when the untargetable spell finishes, your champion will become targetable again, playing the appropriate animation (such as coming up from the pool) even while "stunned".
It's the same reason why Corki's Valkyrie, or Tristana's Rocket Jump still continue to move, even if they are stunned while starting the ability.