Can I use a bus ticket to Laos as a travel booking for my METV application?

Yes

To apply for an METV, you must formally document your need and provide at least two entry and two exit tickets. Enforcement of this requirement varies between embassies and consulates: some require two entries and two exits, while others issue an METV with only a single entry ticket. For a precise answer, it is recommended to contact the relevant embassy to confirm their specific requirements.

Alternatively, you may also choose to stay for 90 days by applying for a 30 days local extension before each departure.

Regarding bus or train tickets, most consulates/airlines would probably accept them as long as it is clear that the ticket is for a journey that crosses a border and the final destination is in the neighboring country.