In our new campaign, our DM allowed us to start at level 3 with two classes so I picked rogue/warlock even though I am going to pursue Warlock beyond level 3. As a level 3 rogue, I am getting sneak attack which gives a damage of 2D6. It works nicely with my Walk Unseen invocation. Only downside is, it requires for me to be within 30 ft of the enemy which breaks my fell flight + plunging shot combo. With plunging shot I can add 1D6 damage as long as I am at least 30 ft above the target. I was wondering if there's a way to increase sneak attack's range that way both plunging shot and sneak attack could work from 30+ ft? That way, my eldritch blast can deal 5d6 of damage while attacking when invisible and flying above 30ft (2d6 from being level 3 warlock + 2d6 from rogue's sneak + 1D6 from plunging shot).
As noted by Wannabe Warlock’s answer, Plunging Shot doesn’t work with eldritch blast.
Sneak attack, however, does. So the question is still valid: how can a warlock enable sneak attack on their eldritch blast from outside 30 feet?
The answer, unfortunately, is “painfully.” Spell Compendium has a swift-action spell, sniper’s shot, which eliminates the range requirement on sneak attack—for one round. That means you have to cast it every turn you want to sneak attack from outside the range. It’s a 1st-level spell, so a wand of sniper’s shot is worth 750 gp, and has 50 charges—that means you are spending 15 gp per turn in order to sneak attack from outside the range. That adds up pretty fast.
But it’s the only solution I can find. There doesn’t seem to be any feat available for this, for example. Crossbow Sniper extends the range to 60 feet, but it only works with crossbows.
The other problem is that sneak attack is pretty poor damage in general—it’s balanced around applying to a whole lot of attacks. Two-weapon fighting or archery with Rapid Shot or Greater Manyshot, so you can attack four or five or six times, getting sneak attack over and over. But you can only use eldritch blast once per round, so you only get that damage once. Dragon Magic does have the eldritch glaive, which lets you (kind of) full-attack with eldritch blast, but it only has a 10-foot reach, so that isn’t really ideal. And your sneak attack is only 1d6 from your one or two levels of rogue.
Personally, I think you should probably just ditch rogue—it isn’t doing enough for you. More levels of warlock would be better than what rogue is doing for you.