I am having trouble maintining the VIP titles that I have assigned to the VIP group in oxide. I set the priorities, but as soon as I assign someone to VIP their player rank disappears. 

That's likely because your player group has "HiddenIfNotPrimary" enabled for its title.

[
  {
    "GroupName": "default",
    "Priority": 8,
    "Title": {
      "Text": "[Nomad]",
      "Color": "#666666",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "Peasant",
    "Priority": 7,
    "Title": {
      "Text": "[Peasant]",
      "Color": "#FFFF33",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "citizen",
    "Priority": 6,
    "Title": {
      "Text": "[Citizen]",
      "Color": "#6666FF",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "knight",
    "Priority": 5,
    "Title": {
      "Text": "[Knight]",
      "Color": "#FF3300",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "lord",
    "Priority": 4,
    "Title": {
      "Text": "[Lord]",
      "Color": "#33FFFF",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "prince",
    "Priority": 3,
    "Title": {
      "Text": "[Prince]",
      "Color": "#FFFFFF",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "duke",
    "Priority": 2,
    "Title": {
      "Text": "[Duke]",
      "Color": "#33FFCC",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "king",
    "Priority": 1,
    "Title": {
      "Text": "[KING]",
      "Color": "#FF3300",
      "Size": 15,
      "Hidden": false,
      "HiddenIfNotPrimary": true
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "vip",
    "Priority": 0,
    "Title": {
      "Text": "[VIP]",
      "Color": "#FF3300",
      "Size": 12,
      "Hidden": false,
      "HiddenIfNotPrimary": false
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  },
  {
    "GroupName": "vip+",
    "Priority": 0,
    "Title": {
      "Text": "[VIP+]",
      "Color": "#FF3300",
      "Size": 12,
      "Hidden": false,
      "HiddenIfNotPrimary": false
    },
    "Username": {
      "Color": "#55aaff",
      "Size": 15
    },
    "Message": {
      "Color": "white",
      "Size": 15
    },
    "Format": {
      "Chat": "{Title} {Username}: {Message}",
      "Console": "{Title} {Username}: {Message}"
    }
  }
]

Merged post

If i make it false, it shows all previous oxide groups (ex.) [Nomad][Peasant]RunawayGuN, it should just be VIP plus whatever current rank they are.

Put the priority of VIP below the ones you want to show and make sure "HiddenIfNotPrimary" is set to false on VIP.

Fixed. Thanks!