You are not setting any values in the variables VehicleType and VehicleVin. Where do the values come from?

Try something like this:

NOTE: You will need to update the code below for the two lines ending with the comment <<<< get value from somewhere

Private Sub CmdVehicleProfileEMail_Click()
On Error GoTo CmdVehicleProfileEmail_Click_Err
Dim VehicleNumber As Integer
Dim VehicleType As String
Dim VehicleVin As String
VehicleNumber = [TFleetRef]

' set the values here

VehicleType = ""   ' <<<< get value from somewhere
VehicleVin = ""  ' ' <<<< get value from somewhere

DoCmd.OpenForm "FQTVehicleProfileEMail", acNormal
DoCmd.ApplyFilter "", "TFleetRef=" & VehicleNumber
DoCmd.SendObject acForm, "FQTVehicleProfileEMail", "PDFFormat(*.pdf)", "", "", "", "Profile vehicle", "Vehicle Type: " & VehicleType & " - VIN " & VehicleVin
DoCmd.Close
CmdVehicleProfileEMail_Click_Exit:
Exit Sub
CmdVehicleProfileEmail_Click_Err:
MsgBox "Your e-mail has not been sent"
DoCmd.Close acForm, "FQTVehicleProfileEMail"
DoCmd.Close acForm, "FQTVehicleProfile"
Resume CmdVehicleProfileEMail_Click_Exit
End Sub