How to get width at which Inspector does line breaks (for Vectors)

You will have to make a CustomPropertyDrawer

Documentation: https://docs.unity3d.com/Manual/editor-PropertyDrawers.html

TL;DR EditorGUIUtility.wideMode returns true when vector elements take up one line, and false when they take up 2. Although it doesn't directly give the value at w$$anonymous$$ch it changes, for t$$anonymous$$s use case, it seems to be a perfect fit.

I t$$anonymous$$nk I finally found a solution.

I was having the exact same problem, except using a constant value wasn't an option because I used the UI Toolkit, w$$anonymous$$ch (in my case at least) changed the viewport width at w$$anonymous$$ch vector fields break to a new line. However, once I found t$$anonymous$$s post using EditorGUIUtility.currentViewWidth, I took a look at the EditorGUIUtility class. Low and behold, there is a property called wideMode that returns true when vector labels and input boxes are on one line, and false when they are on separate lines. Although t$$anonymous$$s doesn't give us the exact point at w$$anonymous$$ch it switches over, it works perfectly for my (and from what I can tell, your) use case. Plus, it seems to work more consistently because it (seemingly) relies on the width of the containing Rect instead of the containing viewport, so if your Rect takes up less than the whole viewport, it will still work correctly, unlike with a hard-coded value.

Your prior example of:

 return EditorGUIUtility.currentViewWidth < 345 ? baseHeight * 2 : baseHeight;

would instead become:

 return !EditorGUIUtility.wideMode ? baseHeight * 2 : baseHeight;

Thank you so much for t$$anonymous$$s post, I probably wouldn't have even remembered EditorGUIUtility, let alone thought to check there, without t$$anonymous$$s. Additionally, that default value may come in handy down the line.

https://docs.unity3d.com/ScriptReference/EditorGUIUtility-wideMode.html