Just my two cents here:

Siroria is not that great here. Try Necro, Juli, Ilambris or Necro, Spinner, Ilambris for a petsorc. Whoever said Ilambris is bad in VMA is just wrong. Ilambris on a sorc that uses a fire and lightning staff is pretty frickin good, especially if you can kite other adds into the aoe. No need to gold out your armor, but you should gold out your weapons.

Also use your sigils. Unless you are going for top 100 for a weekly reward there is no reason not to use them. I have beaten vma over 75 times and still use the sigils on some rounds because why not LOL.

And I might have missed it but what is your CP?

You’re a pet sorc with literally the strongest shield in the game. If you can’t figure out how to spam your shields long enough to stay alive then unfortunately your not a good player....

OP, I struggled similarly - Pet sorc (necro + julianos+ illambris).

tl;dr Use offensive sigils and potions for much higher damage up top.

Changing the color of the damage circles helped me immeasurably; I no longer got blind sided and pinned down on the upper ring. As a result, I could switch to spell crit potions because I only used stam to block and sprint.

I have higher ping, hit the wrong keys a lot, and ftc generally tells me my DPS on this last phase is only 15K or even less. But, this has worked for me: Start last stage, hit my buffs, drop AoEs, pop a spell crit potion. Die to something stupid 4 of 5 times. One in 5 times, the healer, guard and boss are all at the same place and I didn't back into a ghost... adds die in AoE, boss goes up.

Drop AoEs on glowing portal, light attack clannfear, it dies. Get speed and power sigil. Refresh power surge and boundless storm. Go up. (2 in 3 times I die to a ping surprise but when I don't...) Drop blockade and liq light on nearest crystal and trigger familiar, run clockwise (the way the wall moves) a little bit. Block. Go clockwise further to next crystal. Pop spell crit potion and drop destro ultimate on it. Hit boundless again and run like mad counter clockwise to meet the wall, renew power surge and light attack plus AoE the first crystal as you go by (use matriarch heal, too, as needed). Wall explodes and two of the three crystals are gone or very low health. Unload whatever AoEs you have on the third crystal while moving to avoid damage and blocking 2 skulls. Hit boundless storm and power surge. Run Counter clockwise to meet wall and destroy whatever is handy with light attacks. After second wall explosion there should be one crystal left with a little bit of health. Boundless storm, power surge, light attack, heal while running to it. Drop AoEs, on it jump into lava. Crystal dies, boss comes down, yay.

Die 3 out of 4 times in third phase. Finally finish.

Due to internet ping and my own incompetence, it still takes me about 6 hours to get through all of VMA and this last boss often takes me a half hour. I generally force a disconnect every hour or so because of a weird graphics lag that results in inexplicable deaths. Still, I died literally hundreds of times to that very last boss on my first clear. You will get it.