MinMaxSliderAttribute.cs

using System;
using UnityEngine;

[AttributeUsage(AttributeTargets.Field, Inherited = true, AllowMultiple = false)]
public class MinMaxSliderAttribute : PropertyAttribute
{
	public readonly float min;
	public readonly float max;

	public MinMaxSliderAttribute() : this(0, 1) {}

	public MinMaxSliderAttribute(float min, float max)
	{
		this.min = min;
		this.max = max;
	}
}

MinMaxSliderDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(MinMaxSliderAttribute))]
class MinMaxSliderDrawer : PropertyDrawer
{
	const string kVectorMinName = "x";
	const string kVectorMaxName = "y";
	const float kFloatFieldWidth = 30f;
	const float kSpacing = 2f;

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
	{
		if (property.propertyType == SerializedPropertyType.Vector2)
		{
			float spacing = kSpacing * EditorGUIUtility.pixelsPerPoint;
			float indentOffset = (float)EditorGUI.indentLevel * 15f;

			Vector2 range = property.vector2Value;
			float min = range.x;
			float max = range.y;

			float labelWidth = EditorGUIUtility.labelWidth;
			if (EditorGUIUtility.hierarchyMode)
				labelWidth -= indentOffset;

			MinMaxSliderAttribute attr = attribute as MinMaxSliderAttribute;

			EditorGUI.PrefixLabel(position, label);

			Rect sliderPos = position;
			sliderPos.x += labelWidth + kFloatFieldWidth + spacing;
			sliderPos.width -= labelWidth + (kFloatFieldWidth + spacing) * 2;

			EditorGUI.BeginChangeCheck();
			EditorGUI.MinMaxSlider(sliderPos, ref min, ref max, attr.min, attr.max);
			if (EditorGUI.EndChangeCheck())
			{
				range.x = min;
				range.y = max;
				property.vector2Value = range;
			}

			Rect minPos = position;
			minPos.x += labelWidth ;
			minPos.width = kFloatFieldWidth + indentOffset;

			EditorGUI.showMixedValue = property.FindPropertyRelative(kVectorMinName).hasMultipleDifferentValues;
			EditorGUI.BeginChangeCheck();
			min = EditorGUI.FloatField(minPos, min);
			if (EditorGUI.EndChangeCheck())
			{
				range.x = Mathf.Max(min, attr.min);
				property.vector2Value = range;
			}

			Rect maxPos = position;
			maxPos.x += maxPos.width - kFloatFieldWidth - indentOffset;
			maxPos.width = kFloatFieldWidth + indentOffset;

			EditorGUI.showMixedValue = property.FindPropertyRelative(kVectorMaxName).hasMultipleDifferentValues;
			EditorGUI.BeginChangeCheck();
			max = EditorGUI.FloatField(maxPos, max);
			if (EditorGUI.EndChangeCheck())
			{
				range.y = Mathf.Min(max, attr.max);
				property.vector2Value = range;
			}

			EditorGUI.showMixedValue = false;
		}
		else
		{
			EditorGUI.LabelField(position, label, new GUIContent("Vector2 support only"));
		}
	}
}

Min/Max values were not displaying correctly when used within a foldout. This seems to correct that.