I have a 1980s mobile home that's a summer weekend place around Lake Somerville. I'm in the process of renovating it and that started with tearing out some walls. Besides the millions of rat dropping and lizard eggs, I found no vapor barrier on the exterior walls. It was the metal siding, 1in fiberglass insulation, and then wood paneling. My plan was to compress standard faced insulation in (studs are 2x3) and put LP smart siding on the interior walls. Any issues with my thinking? Any worry about the metal siding sweating, even though it's no different from when it was first manufactured.

Clearly no vapor barrier worked fine for 40 years. But why LP smart siding on the inside?

I don't want to mess with the taping and floating of sheetrock and I like the look of LP compared to the 1/8 wood panel sheets.

This might not be THE professional solution BUT
IF this were mine to do ....
This is my core solution
https://www.lowes.com/search?searchTerm=roxul#!
& don't get me started on my love for rockwool ....

The 3" R-value is over 11,7 - hidden for some reason -
https://www.reddit.com/r/HomeImprovement/comments/8h8jjn/roxul_safensound_not_rated_for_exterior_walls_yet/

Although optional for rockwool, I want this chore to be
forever
bulletproof
sleep at night, since you mentioned a concern
... and first install any sort of vapor barrier, the EZiest, the better
If something like this, then I might get some spray glue to hold it up while ti tailor cut it into place
Lowes Item # 1970968

If out of stock, then my fav has always been foil-faced insulation board out then unfaced interior layers
BUT the math invovled to get a few to 2.5-3" tk might be involved

Lack of vapor barrier is standard construction for mobile homes from that era and through the 90's. Mobile homes like that are cheap as **** and poorly sealed. If you're going to be tearing it apart, you might as well pull the metal siding and put on a vapor barrier or put in spray foam to seal it up nice and tight. Otherwise you'll have all kinds of bugs and critters getting into the insulation and everything else.