I was going for the rogue as the main build, with a little bit of the warlock thrown in for an at will ranged touch attack.
If so, I suggest you to multi-class into either Sorcerer or Warmage instead.
Even with only 1 level of these arcane class, you can use 1st-level Lessor Orb spells 4+ times per day, instead of Eldritch Blast. It bypass SR. You can also use some cantrip ranged touch attack spells such as Acid Splash and Ray of Frost.
Then you can go Rogue/(Sorcerer or Warmage)/Arcane Trickster route or just take few levels of Sorcerer or Warmage and then rely on Ring of Wizardry I to double your number of 1st level spell slots.
Either way, Practiced Spellcaster feat will help you bump up damages of ranged touch spells. Unfortunately, this feat does not work for a Warlock.
Increases your caster level and save DCs for (enchantment/against good creatures) and warlock invocations by 1.
Rogue 3/Warlock X is pretty strong, as you get +2d6 sneak attack and evasion. But after a few levels, you will have to decide whether you want to focus on blasting or sneak attack. There is a class that combines ray expertise with sneak attack, I don't remember if it works right out of the gate with Warlock. If not, you could look at modifying it. But it's probably not going to be all that hot.
Was it Spellwarp Sniper from C.Scoundrel? Just a guess, don't have the book around right now.