[Vrul]

Give this a try:

local BUTTON_SIZE, BUTTON_SPACING = 50, 1

------------------------------------------------------
--[[              Bar Setup                       ]]--
------------------------------------------------------
local barFrame = CreateFrame("Frame", "XBar_Vehicle", UIParent, "SecureHandlerStateTemplate")
barFrame:SetSize((BUTTON_SIZE + BUTTON_SPACING) * NUM_ACTIONBAR_BUTTONS, BUTTON_SIZE)
barFrame:SetPoint("CENTER")

local background = barFrame:CreateTexture(nil, "BACKGROUND")
background:SetColorTexture(0, 0, 0, 0.5)
background:SetAllPoints()

barFrame:Execute([[
    ActionButtons = newtable()
]])

------------------------------------------------------
--[[                 Button Setup                 ]]--
------------------------------------------------------
local buttons = { }
for id = 1, NUM_ACTIONBAR_BUTTONS do
    local button = CreateFrame("CheckButton", "$parent_Button" .. id, barFrame, "ActionBarButtonTemplate")
    button:SetID(id)

    button:SetSize(BUTTON_SIZE, BUTTON_SIZE)
    if id ~= 1 then
        button:SetPoint("LEFT", buttons[id - 1], "RIGHT", BUTTON_SPACING, 0)
    else
        button:SetPoint("LEFT", 5, 0)
    end

    button.NormalTexture:Hide()

    barFrame:SetFrameRef("button", button)
    barFrame:Execute(([[
        ActionButtons[%s] = self:GetFrameRef("button")
    ]]):format(id))

    buttons[id] = button
end
barFrame.Buttons = buttons

------------------------------------------------------
--[[                 Secure Stuff                 ]]--
------------------------------------------------------
barFrame:Execute([[
    self:SetAttribute("frameref-button", nil)
]])

barFrame:SetAttribute('_onstate-page', ([[
    newstate = tonumber(newstate)
    self:SetAttribute("actionpage", newstate)

    local pageOffset, hasAction = (newstate - 1) * NUM_ACTIONBAR_BUTTONS
    for id = 1, #ActionButtons do
        if HasAction(id + pageOffset) then
            ActionButtons[id]:SetAttribute('actionpage', newstate)
            hasAction = true
        else
            ActionButtons[id]:SetAttribute('actionpage', nil)
        end
    end

    if not hasAction then
        self:SetAttribute("state-page", nil)
    end
]]):gsub('NUM_ACTIONBAR_BUTTONS', NUM_ACTIONBAR_BUTTONS))

RegisterStateDriver(barFrame, "page", ("[vehicleui] %s; [overridebar] %s; 1"):format(GetVehicleBarIndex(), GetOverrideBarIndex()))
RegisterStateDriver(barFrame, "visibility", "[vehicleui] [overridebar] show; hide")

OverrideActionBarLeaveFrameLeaveButton:SetParent(barFrame)
OverrideActionBarLeaveFrameLeaveButton:SetAllPoints(buttons[12])
RegisterStateDriver(OverrideActionBarLeaveFrameLeaveButton, "visibility", "[canexitvehicle] show; hide")

If that works I can modify it so you only need the one bar and not separate main, vehicle, override...