Neither.
You have used incorrect syntax for both. If you want to use getObject method then you should fully use getObject, getLink & setValue syntax.
getObject... getObject("Scenes[optionMenu].SceneValues[Music_Volume]"):setValue(VValueInt, 0)
shorthand... Scenes["optionMenu"].SceneValues["Music_Volume"].Int = 0 -- or...
Scenes["optionMenu"].SceneValues["Music_Volume"].ValueInt = 0 -- or...
Values["Music_Volume"].Int = 0 -- why bother direct linking if you probably only have one value with that name?
P.S: it is possible that you may be able to mix & match shorthand Lua with the getObject method, but for sake of consistency it's better to stick with one or the other. In the next release, shorthand is a lot more complete & you can access pretty much all data structure fields & tables with it, unlike the current public build.