The answer depends on whether any LVDS input pair is used or not in the bank.
1. If a DDR1/2/3 memory interface is implemented in a LatticeECP3 bank and also includes at least one LVDS input pair, the common mode termination is enabled. In this case, it is suggested that you make the unused VTT pins AC-coupled to ground. When they are AC-coupled to ground, a capacitance 0.01uF or 0.1uF is generally suggested. Note that the value of a capacitor for VTT AC-coupled to ground should also depend on the LVDS driver specification. If the driver does not allow high capacitance on the common mode termination, a 0.01uF capacitor would be a better choice.
2. If no LVDS input exists in the bank, the unused VTT pads can be left unconnected. You must turn off the internal VTT termination. Make sure that the TERMINATEVTT preference is set to OFF.