This is a known issue with Valkerian Warfare. Their mod messes with the core Minecraft collision system, which MTS uses to detect blocks below vehicles. For some reason, they are not returning the correct hitboxes for blocks that Vanilla Minecraft/Forge does, resulting in MTS not finding any collidiable blocks below the vehicles. It is up to THAT mod to fix their hacking in Vanilla code, not MTS to accommodate said hacks. There is already an issue on that mod's GitHub, please reference it:
ValkyrienSkies/Valkyrien-Skies#170

idk why github didn't notice me about new comments after my comment, damn

and @Dvoric well, idk if you are new to Modded Minecraft, but in these cases:
-you first should do a Binary Search (take half of your mods away of the Mods folder, run minecraft and test if the issue is fixed, if it is still present then repeat until you end with the only guilty mods, if not, then the guilty mod was in the mods you removed)
-And second, you should open a new issue and mention this one copying the URL rather than asking here, this issue is closed and as you said, it is not related to yours

But most important, you may prefer to ask first in the Discord Server before: https://discord.gg/KaaSUjm
maybe is something trivial, if not, then open an issue (although i never saw this incompatibility with any other mod)

Hi.

I am NOT using "Valkyrien Warfare/Skies" ... but it certainly seems like I am experiencing the same exact problems as mentioned above ...

I place a vehicle frame in my world and it clips into the ground after a few moments, until it eventually starts "breaking" the blocks beneath it, and slowly sinks to the bottom of the chunk.

It's pretty funny to watch, and I'm new to your mods, but I didn't think it was normal behavior so I thought I'd check out issues and let you all know. =)

I am using the latest version of IV, and the "official" vehicle packs, and the machinery pack. I also placed the car frame in creative mode. I haven't tried survival yet. I am using 250+ mods. I make my own modpacks. If any of that info helps?

So I assume there's a conflict somewhere but I can easily disable mods. I'm sure I will eventually find the problem.

If you can help spot the problem quicker by looking at my mod list or whatever then let me know. I'm thinking it's most likely gonna be terrain generation or biome mods but I know nothing. I don't think I have anything that's even remotely close to Valkyrien.

Thanks.

So the fact the vehicle is breaking blocks tells me it's not the same issue. This issue resulted in vehicles just phasing though the ground and not interacting with anything. Your issue sounds like #959. If so, we should probably continue the conversation over there.

This is a late comment, so you might know how to fix this issue:

I know why the vehicle falls through the ground, you didn't add wheels to the vehicle. That's probably why it falls through, hope that clears the issue.

No. That is not why it falls through. It falls because there is nothing for MTS to collide with due to VW messing with block physics. Even if you were to add wheels, it would still fall down, through all blocks, and into the void beneath the world.

Man, mods do mess some things up.

Marking as closed as of ValkyrienSkies-1.12.2-1.0-alpha-1. Appears a code re-write occurred that fixed this collision issue.

I will investigate this issue. Thanks for redirecting people to us.

Update:
MTS: Immersive+Vehicles-1.12.2-19.11.0
VS: valkyrien-skies-1.12.2-1.1.0

now vehicles are practically functional, but there are some visual bugs and when you try to get out it appears that you die from fall damage (it even hurt you in creative)

Videos:
https://streamable.com/1m7vpk (MTS and VS only)
https://streamable.com/9nesys (A bunch of mods)

I managed to modify MTS vehicles to collide with ships and get dragged with ships.

Maybe I'll make a pull request to fix it.

I'm going to say no to that, simply due to the fact there's no reason MTS vehicles should, in normal circumstances, be moving into the ground downwards. The reason for the change in this behavior is because I have internal variables that are separate from the main entity that control physics and rendering. So the MTS physics engine knows there's ground below it and knows the vehicle didn't move, but it's being moved into the void anyways. This is why they turn dark (due to no light), and they dismount you into the void (because the entity is down there). In general, VS shouldn't be touching the vehicle entity at all if it's just sitting on the ground. Especially if there aren't any ships in the area.

It kind of works https://youtu.be/ORvdkAXYJEc

It looks strange. Now that I have both two mods installed( MTS 1.12.2--19.14.0; VS 1.12.2--1.1.6, and I don't notice anything unusual when I'm driving a vehicle from MTS, or even when I'm getting in and out of the vehicle. Is there something I'm missing?

Oh yes. Just now when I used VS 1.1.0, all the vehicles I put became black. And when I change VS back to 1.1.6, all the vehicles are back to normal.

That's a different version that what we've seen issues on. As I and @gatoborrachon said, it was VS 1.12.2-1.0.0, and 1.1.0 that had issues. Could you try again with 1.1.0 and see if the issue re-appears? It could very well have been fixed again.

Awesome! I'll mark this issue as re-solved as of VS 1.1.6. Hopefully it stays solved!

Re-opening, as the next release after that alpha re-introduced the issue. From "Valkyrien Skies 1.12.2-1.0.0" onwards, V18 vehicles again fall through the ground into the void. For V19, the vehicles don't fall into the void visually, but they do so in Minecraft code. Because of this, they render black, and when a player dismounts them, they are put into the void where MC thinks the vehicle is.

Want to mention that unlike the first comments, the source of the issue isn't that vehicles aren't seeing collision blocks. Rather, they are seeing the collision blocks, and preventing themselves from moving, but VW is forcing the vehicles to move downwards from their end. It's why the vehicle renders above the world, but you get sent below it when you dismount.