Both sound cool (not as cool as the Chilly Tentacles, but...

)
Wall of Fire and the Chilling Tentacles cover the same spot - controlling the area by restricting safe movement for enemies (but they can hinder the party). So, Wall of Fire might not turn out so important for your character, but it offers more overall flexibility. If you have no way of dealing fire damage yet, it is a very useful choice, and its "destruction" effect on burning away opponents is nice, too (even if usually inconsequential).
Painful Slumber is basically a "Save or Die" effect that you can do all the time. Many opponents might turn out immune or very resistant to it, since it's a sleep effect.
With Vitrolic Blast, you bascically have enhanced the utility of your Eldritch Blast that makes it useful against most opponents (assuming that you also have one or two of the shapes of the lover-type invocations).
The risk with Painful Slumber is that it's an all or nothing effect. Eldritch Blast at your level is usually a guaranteed effect, which stacks with your party efforts to take down an opponent. If you have a very high charisma score, Painful Slumber is a good choice. But if you neglected your charisma so far, Painful Slumber forces you to improve it.