Because discussions over viltrumite's weakness to heat I decided to bring some examples from the comics. I'll put a spoiler because it would be a spoiler for the show. After the sixth image I can't hide the spoiler from you

**First page we have his heat based enemies (text from the official handbook of the invincible universe):** From top to bottom:

Furnace ( issue 19):  His body heat is around 400 °C and with his attacks can reach 1500 °C.

\>!Doc seismic!< (issue 107): No official temperature but if we assume it's regular magma it should be between 700 °C and 1200 °C. His powers are related to Volcanikka

Magmaniac (issue 47):  his lava form can reach temperetures from 700 °C to 1200 °C. It should be noted we don't really see him hitting mark in the comics (only in the cover and in the show).

I think this is very consistent and shouldn't be controversial. 

**Explosions:**

Second image is when he punches a nuclear weapon causing it to explode (Issue 52). The nuke used by the mauler twins. No known temperature. It seems to take place in space so heat transfer would be different, we don't know how powerful was this explosion and it didn't damage his suit so hard to properly define.  

Third and fourth images (issue 80): Bombs that destroyed >!Las Vegas!<. The bombs completely obliterated the city and are a new technology so we don't know how they work. It was stated at least 50 bombs were needed for this.

If we want to have a lower end we could assume a heat based on the little boy bomb. Wikipedia tells me it reached a fireball temperature around 6000 °C

Page 5 is atmospheric entry followed by going through viltrum (issue 75).  We don't know how hot these would be but we could try comparing it to earth's core and atmosphere for a reference. 

Assuming Viltrum's core is similar to Earth's it would be roughly  5700 °C.

I think we could argue that the 5000-6000 °C doesn't seem to hurt if it's a short exposure.

The sixth image is a representation of the sun's structure and after that we have spoilers  

Now we enter into a high spoiler territory (issue 140):

\>!The fight with Thragg at the sun!<

First we would need to define where they are. 

\>!We see them fighting over a yellow surface full of waves and bubbled, even being hit by an eruption from the star. This would suggest we are seeing granules forming from convection.!<

Here some definitions would be good. What we call the surface of the sun is a layer called the photosphere and it's actually part of the sun's atmosphere. Below that we have the outermost layer of the sun, the convection zone.  Like the name says it has heat transfer through convection it generates the granules in the sun, like bubbles in boiling water.

I believe the fight takes place in the photosphere with dips into the convection zone. 

Photosphere temperature: roughly 5700 °C

Convection zone: a gradient going from 2 million to 5700°C. 

If you disagree with this I'm all ears.

There is a question of density and heat transfer that I don't know how to calculate but it should be known. There's also the fact that we don't know how deep they went or how long they stayed there which makes it harder to measure their exposure.

I think it's good to point out that while the plasma granule didn't seem to hurt them a short contact with that plasma could injure them. 

Either we can assume 5700 °C doesn't hurt them and they need extra heat from the convection zone to be injured or we assume the extra density was enough to increase the heat transfer and injure them. Maybe a combination of both.

I tried to be conservative here and not go for the highest temperature possible but feel free to calculate a high end for these.

Invencible (Invincible comic) heat resistance