While you can reset settings for Blender which is easier to do, you might want to preserve all the personal preferences. They will be all removed with Load Factory Settings and it should be used if other ways won't work.
The shortcut for toggling Vertex Paint interaction mode on Left Mouse was probably created by mistake from the changing modes menu in the 3D Viewport footer, when you misclicked on menu entry with RMB (which opened a context menu), then clicked on New Shortcut and next click was considered as input for the new shortcut.
To remove it open User Preferences > Input. Switch it to Key-Binding and enter "left mouse" as search word (without quotes).
Scroll the window down, to the Object Non-Modal section. There will be Set Object Mode setting with Vertex Paint as parameter and LMB as shortcut to invoke. Delete it.