Absent a hit to a vital organ or cns, you need blood loss. The more the better. Expansion creates a bigger wound channel... which is to say causes more internal bleeding / faster blood loss.
Penetration alone isn’t all that. The 62 gr penetrators (steel core and tungsten) in Iraq and Afghanistan just punch little holes. The old 55 grain tumbled. That tumbling also creates a bigger wound channel, and more internal bleeding / faster blood loss.
Actually no, the expansion channel has shown to cause almost no damage to tissue. I'm sure the expansion is incredibly painful but that's just from having a massive cavity suddenly open up inside you. There is literally VERY little damage. The only tissue that is actually damaged by a bullet is the actual tissue that comes in contact with the bullet
If you have ever seen a deer or other animal shot with a Soft point or HP, you would see a 2 to 3inch track where the tissue is totally destroyed. That 2 to 3in diameter track of destroyed tissue is what was actually in contact with the mushroomed bullet face. The larger diameter is caused from the bullet hitting the tissue and pushing into it, stretching the tissue before the bullet actually tears through it. Even though that tissue is completely destroyed, its still pretty survivable (provided as you said, minus a hit to major organ, cns, or a major artery) because the body is self sealing. Torn tissue swells and prevents blood loss. That is why a deep knife wound is much more dangerous than a bullet hole. Cut tissue doesn't swell enough to prevent the loss of blood. Which brings me to the part you said about the 55grn M193 used in Vietnam.
M193 was said to be so devastating from tumbling. That could be further from the truth. In fact it was a closely guarded secret by the DOD. DOD simply let the tumbling rumors stand. The truth is, it was the fact that high velocity bullet shattered on impact and the fragments cut through the tissue and organs. The sharp copper jacket fragments created multiple track wounds of cut tissue the bleed much more than the lead fragments did. Vietnamese often bled out after being shot with M193.
M855s steel core wasn't nearly as good as M193. The steel core didnt deform like M193 and so the jacket often didnt break up into as many fragments. Dont be fooled though, M855 is still fairly effective. I have my own personal experience with M855