http://www.newvegasnexus.com/downloads/file.php?id=42180 is catching up, but http://www.newvegasnexus.com/downloads/file.php?id=35998 (possibly combined with Fellout) are the main weather mods. Compare the screenshots, as Nevada Skies and Project Reality go for quite different effects, particularly in terms of colour saturation. The Director's Chair, and the mods it contains, are useful for further tweaking. All of these weather mods give you darker nights, and Nevada Skies allows you to adjust the level of darkness in-game.
For darker interiors, http://newvegasnexus.com/downloads/file.php?id=35794 is a nice addition to general weather mods like Nevada Skies.
Not sure what effects these mods have on the orange fog in Dead Money. The Dead Money patch for Nevada Skies URWLified says it does this:
NSkies URWLifiedDM.esp is actually URWL for Dead Money and does not need a amster, it also works without NSkies URWLified.esm, so that one is standalone, it does not make Dead Money realistic, it just edits the ambients to be grey so the red colors only come from the sky and the fog in the streets, it is still very red and at night it is dark, unlike the normal Dead Money.
The Vault says light levels do affect sneak attempts, assuming it hasn't changed between F3 and NV:
Light level: The player is more difficult to see in dark areas or at night if outdoors. The Pip-Boy light, while active, will make the player much more visible. Likewise, glowing weapons such as the Shishkebab, plasma rifle, Shock Baton and Jingwei's shocksword can give away the player's position. Tesla armor is especially difficult to sneak in due to its high weight and many bright lights. Light effects can be partially countered by a stealth field, but never completely.