I think the rub is that the military is inherently political, but isn't supposed to be partisan, and people often mistakenly conflate the two. Every time a service chief or a legislative liaison goes to the Hill for budget stuff, it's political. When we make tactical, operational, or strategic choices, it's political. War is politics. Clausewitz etc. etc. The issue is when you mix service with promoting a partisan candidate or cause.
Likewise, impeachment is inherently political (see Federalist 65) but isn't intrinsically partisan. The founders naively thought political parties wouldn't become a thing but a decade before the first partisan presidential election (Federalist Adams vs. Democratic Republican Jefferson in 1796) they wrote impeachment into the constitution on the theory that the competing ambitions and different constituencies/time horizons of a non-partisan House (elected by the people every 2 years) and Senate (elected by state legislatures every 6 years) would equip them to keep a chief executive from abusing his power. They didn't anticipate a lot of trends that subverted that structure: the evolution of political parties, direct election of Senators, indirect popular election of the President (i.e. EC changing from a deliberative body to a ceremonial body that ratifies the results of 50 statewide presidential elections), mass media, and a series of presidents (Andrew Jackson, Teddy, Wilson, FDR, JFK, LBJ, Reagan, etc.) shifting the CG of the political parties away from their congressional wings and toward their presidents/presidential candidates. So today members of Congress are incentivized to stick by THEIR president in any impeachment proceeding, lest they offend their primary voters, and so much for competing ambitions between the executive and legislative branches... And impeachment is seen as partisan like everything else Congress does, e.g. budgeting/appropriations, oversight, confirmation of appointees, etc.
All that being said, just because the budget is politicized doesn't mean the CSAF doesn't show up to testify in favor of the Presidential Budget. He's just supposed to say "Vote to fund this because the AF needs it to fulfill the NDS and execute our OPLANs," not "Vote for this to send a message that President Trump supports making our military great again and to own the libs." Likewise, if a military member gets subpoenaed to testify to what they know in an impeachment inquiry, well, they kind of have to do so. As long as it's "I saw this and heard this on this date, and then this happened, etc" and not "I Colonel So and So say Trump has to go. Medicare for All." And if it's service protocol that you testify in uniform, then you testify in uniform.
Those who are talking about Vindman substituting his judgment/views over those of the head of the executive branch... Well, they would be right if we were talking about the "high criminalization and misdemeanoring" of mere policy differences. But we're not. Presidents including Trump have disagreed with the consensus of their interagency process before. Presidents including Trump have blurred the org chart before, e.g. Obama making Dick Holbrooke his Afghanistan/Pakistan guy in lieu of working through his ambassadors to those countries, = not all that different from Trump making Jared Kushner his Middle East Peace Czar or even in this case having EU Ambassador Sondland work Ukraine issues. What is new and different here is asking a country to play ball with someone outside government, Rudy G, as your personal attorney and representative of your reelection campaign, in order to receive aid that was lawfully appropriated by Congress. The key factors in the allegation are that appropriated funds were being withheld until Ukraine cooperated with Trump's personal representatives in promoting false stories about a political opponent. If Trump had said through Taylor, Sondland, Pompeo, the State Department janitor, or anyone else on the federal payroll, "I will oppose and/or veto future funding for aid to Ukraine until my Justice Department tells me you are cooperating with it's investigations into XYZ," that's in the realm of policy differences. When you circumvent your ambassadors and attorney general, and say "Get with my private lawyer and coordinate a public statement saying you're investigating Joe Biden for something that didn't happen and that you're investigating yourself for the DNC hack that my intelligence and law enforcement agencies all say the Russian Soviets did, or you can't have the money Congress appropriated and I signed into law," that's a potential abuse of power. (And remember, the Supreme Court declared the line-item veto unconstitutional back during the Clinton years, so the president does not have carte blanche to not spend appropriated funds.)
A much shorter comparison: Disagreeing with your own appointees about the wisdom of selling arms to Iran to encourage Hezbollah to release hostages, not a crime. Using the proceeds to pay for things Congress banned you from paying for (Boland amendment) in a bill you signed, maybe a problem. And when that happened, military members of the NSC got called to testify about it and they testified in uniform.
[FWIW... I have much less of a problem with doing accounting tricks to pay for fighting the commies in Latin America than I do with holding "fight the commies in Eastern Europe money" hostage to your reelection campaign. But that's just my two cents.]