According to Vi Rep:
If you recharge with a voice/data pack, the benefits will co-exist with your existing pack. If both the packs have outgoing & data service validity, the one with the higher outgoing & data service validity will be considered.
According to another rep:
If you’ve done more than one recharge, both recharges would be clubbed together. Benefits and validity will be extended to ensure you get the best value for your spend.