Threat born of insecurity
Then again it was also reasonable then, and even when the Senate was restored in 1941, to assume presidents and veeps would operate as a team. In the days when party affiliation counted for something, and party conventions put together tickets with regional as well as national appeal, chances were rivalries could be set aside for the duration of the term. It also meant that the electorate, as a rule, also viewed candidates for the presidency and vice presidency as a ticket. Voters could—and still can, and indeed, nowadays, they generally do—vote split tickets. But on the whole, they did not.
That is, until the party system started to break down: it’s no coincidence that the first presidential election that produced a minority president—Carlos P. Garcia, in 1957—was also the first election in which the president and vice president came from different parties: Garcia was a Nacionalista, Diosdado Macapagal was a Liberal. The two parties had their last competitive conventions in 1965 (for the Nacionalistas, when Marcos defeated former Vice President Emmanuel Pelaez, who up to 2016 was the highest-ranking Mindanaoan elected to national office) and 1969 (for the Liberals, who nominated Sergio Osmeña Jr.).
Since then, presidential and vice presidential campaigns have been personal coalitions. Party affiliation is more of a financial question (as parties are bankrolled, or headed, by business interests) and a vehicle for logistical requirements like precinct watchers. With no incentive to think in terms of a ticket, candidates, supporters and the electorate have little concern for whether the eventual winners will get along, or not. They may even harbor a kind of malicious glee in ensuring the opposite by voting for candidates they know will rub each other the wrong way, out of a kind of instinctive sense of checks-and-balances.
What is odd is that when the presidential system was modified by the framers of the 1987 Constitution, no one seems to have bothered to ask why it was that Filipinos voted for the president and vice president separately in the first place. In 1935 when that decision was made by the framers of an earlier constitution, voting for the two top positions separately was part of the nation-building exercise of the era. Knowing they were instituting national elections for the first time, it was crucial to ensure that both the president and the vice president (whose job it would be to assume the presidency in case of emergency) had iron-clad mandates the people would respect. Having a two-party system ensured mandates would be majority ones; voting separately ensured no vice president could be brushed off as lacking an individual mandate.
Since 1987 veeps are either a threat to neutralize, or, in the rare exception when a ticket won, to fill with a person so unsuited to be president, their holding the job would in turn help the president keep theirs. What is unique, now, is that there is no strategic patience on the part of the ruling coalition. To dangle impeaching the veep is to reveal they lack confidence in the future.