PO in vacation
what happens to sprint planning when the p.o. is on vacation?
2 week sprints and 4 weeks vacation
There will presumably be a risk to product value optimization.
Are the team confident that they are sufficiently prepared to manage that risk during the Product Owner's absence? If not, why not?
There's a recent thread about the absence of a Product Owner that is relevant.
The Sprint Planning, and the other Scrum events, proceed as usual on their regular cadence.
What to do depends on the team, though. Any single person that the team depends on is a risk that should be mitigated. In this case, I'd be interested to understand if the Product Owner delegates some or all their work to others. If so, this person may have the insights needed to stand in for the Product Owner. Otherwise, there may be someone else with information about the product, the current Product Goal, and the stakeholders to fill in.
As always, the best thing to do is to talk to the team. There may be other stakeholders in the development organization who can help offer suggestions or support. The Scrum Master is a good person to facilitate any discussions on how to handle this. And, since it's a vacation, hopefully it's planned in advance to include the Product Owner in these discussions.
The wisest course of action, as Thomas noted, is to consult the team before leaving.
Although the PO is always responsible for the Product Backlog and communicating the PBI items to the developers, this does not preclude him from assigning his duties to someone else while he is temporarily away for any reason.