Can a vice-president vote for his successor in the Senate before becoming president under the 25th amendment?

How would tie votes in the Senate be resolved if there is no VP due to the VP assuming the presidency?

The resolution of a tied vote is the same whenever the VP cannot or simply does not vote, whatever the reason. Possible reasons include

1. as noted in the question, there might be no vice president;
2. the vice president might be out of town or otherwise unavailable; or
3. the vice president might simply choose not to vote even while presiding over the senate.

In any case, the resolution is quite simple: the senate always votes on questions that may be answered "yes" or "no." For the question to pass, a majority of the votes must be "yes" votes. If the yes and no votes are tied, there is no majority of "yes" votes, and the question does not pass.

Can a vice-president vote for his successor in the Senate before becoming president under the 25th amendment?

No. The successor must first be nominated by the president, so even if we assume (contrafactually) that there were some time after the president's death or departure during which the vice president remained vice president, there could be no vote in the senate for the VP's successor because there would be no president to nominate anyone. The successor cannot be nominated until after the vice president has become president.

Or would the vice-presidential vacancy only happen after the ascension, at which point the vice-president no longer has the Senate vote?

Yes, this is how it works. To be more specific, these events must occur in this order:

1. The president dies, resigns, or is removed from office.
2. The vice president takes the presidential oath of office.
3. The former vice president nominates someone to be the new vice president.
4. The senate votes on the confirmation of the nominee.

There would be no vice president in that situation so the vote would be a tie. Unless there was able to be some sort of deal made another candidate would need to be chosen to fill the seat of vice president or it would just remain open until the next election.

https://prologue.blogs.archives.gov/2017/02/10/the-25th-amendment-succession-of-the-presidency/

There have been 16 times, totaling 38 years, that the Vice Presidency has been vacant. This has been due to the death or resignation of the Vice President, or when the Vice President has assumed the Presidency after the death or resignation of the President.

https://www.senate.gov/artandhistory/history/common/briefing/Vice_President.htm

In 1947, after the vice presidency had been vacant for most of a presidential term, Congress again changed the line of succession. Concerned that cabinet officers had not been elected, it named the House Speaker as the first official to succeed if a president died during a vacancy in the vice presidency, followed by the president pro tempore.

It should be noted that there was not a VP from April 12th 1945 until January 20th 1949 when Truman took office after the 1948 election.

https://en.wikipedia.org/wiki/List_of_vice_presidents_of_the_United_States