How do you know that the exchange rate will adjust accordingly ? It may not, markets don't always act rational, even more so when everyone are told that 9/10 of their wealth will be wiped out
The exchange rate of VESTs to SP is not determined by market forces. It is hard coded to change over time in order to dilute the supply of SP.
You can check the current rate at
https://steemd.com/distributionCurrently it is
1M VESTS = 216.819 STEEM
Note that what is called SP is simply VEST balance denominated in STEEM equivalents.
There are mechanisms in that act as automatic stabilizers in the case of market volatility, by the way... We could eventually discuss those. They're very clever.