When there is a rebalance event, you will receive a notification that a rebalance is needed. You can choose to ignore or accept this notification. If you accept, you will be taken to the transaction confirmation page on the website or app where you can see what buy/sell transactions will occur. You can then accept and your allocation will match the current most up-to-date allocation.
If you do not accept the rebalance, your allocation will deviate from the default. Therefore the performance chart within the Vest details will not reflect your performance anymore (since the chart only reflects the default allocation). However, the 'Your Position' section will always give you the most accurate returns.
Note that rebalancing just changes the allocation (the mix within the portfolio). It will not increase the total amount you invest in a Vest.
For example, if you had invested 1,000 USD in a Vest that comprises 2 stocks, A and B (in a 60:40 allocation), and there is a rebalance that will change the allocation to stocks A and C (in a 50:50 allocation and if, at the time of the rebalance the value of the investment is still 1,000 USD then the rebalance actions will be:
Sell some of A to bring allocation to 50%
Sell all of B
Buy C to fulfil 50% allocation
After rebalancing, the total investment is still 1000 USD (no new cash infusion will be deposited)