At $9.50/mo lease, it will take four years of lease fees to pay off the current VOOM STB. But instead of leasing for four years, you want VOOM to turn around and swap it out for a $800-$1000 DVR, which will take 7+ years to pay off at $10/mo.
Even at $20/mo DVR rental, you'd have to be a subscriber for three to four years before the rent would pay off the box. Yes, they make money on the programming, but margins are not that significant (if Dish and DirecTV are any indication) on the $39.95/mo package. Dish had to average well over $50 per subscriber to turn in positive EBITDA, and that's without leasing an expensive HD DVR to any customer, let alone every customer.
If the DVR rental were only available to VaVaVoom subscribers at $19.99/mo or $24.99/mo, then maybe. But no way will they possibly make it available at $10/mo for $39.90/mo subscribers.