It would be best to get a pair of matched batteries. The two batteries you have at the moment are different capacity - the VTC5A is 2,500 and the VTC6 is 3,000mah, if you use them as a pair the VTC5A will discharge faster than the VTC6 and be at a lower voltage and have to work harder as it will have to provide more current - the chip in the mod may be able to balance charge (personally I wouldn't rely on it and would advise buying an external charger) but it cannot balance discharge two batteries in series. The mod will work with mismatched batteries but I wouldn't recommend it.
When it comes to batteries it is best to buy the ones that suit your vaping habits - if you are likely to be vaping up to 90 watts a pair of VT6, Samsung 30Q or similar 3,000mah 15 to 20 Amp batteries would be a best buy, if you are likely to go higher than this you would be safer with a pair of 25 Amp VTC5A , Molicel P26a, P28a or similar. The Amp rating is more important than the mah capacity for high powered devices - if in doubt go for the higher Amp batteries.
Don't be tempted to buy 3,500mah batteries as they are designed for low power use only and are totally unsuitable (and unsafe) in a high power device.
Vaporesso Luxe - good choice.
