yes you should be able to, since the stacks blow up only once the damage is applied, which is however long it takes the tendrils to reach them.
also, idk about elixir of sorcery, haven't had the chance to try it on him yet, but what I WILL say is that AP varus is a great counter to the tank meta, since his W gives him bonus magic damage on hit that scales with AP, meaning that getting AP still increases yout auto atk damage, AND it boosts the damage your blight stacks do when they blow up. This means that your skills (Q and E) are going to be there just to blow up stacks for a good chunk of damage. I played AP varus last night and our team was pretty much AD (a panth mid, xin jung, sion top) and so i decided to go ap cuz troll, and I was blowing up the enemy renek pretty damn fast even with the armor they had. I had around 400-ish ap and was doing around 13% hp in magic damage PER blight stack (not applying MR of course)which is 39% per 3-stack blowups, PLUS the magic damage procs from W boosting my autos, PLUS the MR I was stealing with my wit's end.
Here's a quick look at the match history if ya want.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1813687698/208450390?tab=overview
Started with W for farming, then Q for poke, then W again though i should have gone E for slows and more blowups, but yeah maxing W first as ap varus so you can melt % hp, then I'd max E second because I want faster cooldown on that slow aoe.
build zhonyas first for ap and armor, since fighting against AD MF. then went with witt's for atk speed and MR shred, rabadons for more pwer and then hurricane for the multi-procs, more atk speed, and that teamfight ult for pewpew damage. Lucid boots cuz i wanted cooldowns, the faster my skills are back up, the faster I can proc again. Was considering geting ROA last for sustain cuz karma Q's were annoying the hell out of me, or maybe a liandry's for more magic pen to get closer to true damage. none of them had any REAL MR, zed had merc treads and karma had athenes and that's about it. renek's fat armor stack did nothing for him against me.