Technically, SP holders are holders of VESTS. Your SP is calculated by multiplying the amount of VESTS by 483.452 (Current value of 'steem_per_mvests' on steemd.com). By slightly increasing this value overtime all VESTS holders see their amount of SP increasing.
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Interesting... So steem_per_mvests is what increases, which increases the value of held VESTS... Thakn you so much for your insight!