This problem might admittedly be because of the way I'm trying to use the plugin.

Anywho I'm trying to get it so being near a TC outside is 50% decay reduction.... and inside is 25% - adding up to 75% when near TC and inside.

However, instead it seems vehicles are being given COMPLETE reduction when either of these conditions are met. I'll go ahead and share my config and wait for some input.

{
  "Enable permission": false,
  "Vehicles": {
    "Duo Submarine": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Hot Air Balloon": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Kayak": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Minicopter": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Modular Car": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "RHIB": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Ridable Horse": {
      "Allow the plugin to influence decay": false,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Rowboat": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Scrap Transport Helicopter": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Sled": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Decay interval (seconds)": 60.0
    },
    "Sled Xmas": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Decay interval (seconds)": 60.0
    },
    "Snowmobile": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Solo Submarine": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    },
    "Tomaha": {
      "Allow the plugin to influence decay": true,
      "Decay multiplier while inside": 0.25,
      "Decay multiplier near tool cupboard": 0.5,
      "Protect from decay after recent use (minutes)": 60.0,
      "Decay interval (seconds)": 60.0
    }
  }
}

That approach should work, but I think your numbers are wrong for your goal.

The multipliers stack multiplicatively, not addictively, so 0.5 * 0.25 = 0.125 (87.5% reduction).

It you open the .cs files you can uncomment the debug variable at the top (remove the slashes) to enable debug mode to see what's actually happening.

Most likely, you are seeing complete reduction because you recently spawned or used the vehicle. Debug mode will tell you this.

Actually I was not aware that's how it worked. Note I'm not testing on vehicles I spawn, smarter than that ;) Just checking random player vehicles. 

So basically I multiply the two settings by eachother is what I am understanding? 

The protection after use feature is vanilla. The plugin simply makes the duration configurable. Maybe I can rename the config option to "Nullify ..." instead of "Protect ..." to reduce confusion.

Yes, multiply the multipliers together to determine the actually multiplier used.

WhiteThunder

The protection after use feature is vanilla. The plugin simply makes the duration configurable. Maybe I can rename the config option to "Nullify ..." instead of "Protect ..." to reduce confusion.

Yes, multiply the multipliers together to determine the actually multiplier used.

For the sake of confirming I understand.... what would you put into my config for my desired result of 50 near TC + 25 more inside?

0.5 and 0.5

When only one of those criteria is met, you will get 0.5x decay. When both are met, you will get 0.25x decay.

WhiteThunder

0.5 and 0.5

When only one of those criteria is met, you will get 0.5x decay. When both are met, you will get 0.25x decay.

Got it! I definitely understand now. Thanks for all the help