Even if that player performs worse than what Vargas would perform, its still more valuable to play that player, simply to evaluate that player and prepare for the future. Which is still the goal of the 2024 season. There is no reason to give Vargas any substantial playing time. He is not a factor in the Nationals rebuild. He is only getting this contract because he is cheap and he has a positive attitude.
If he's healthy, start Kieboom next season. If not, start Alu. Garcia starts at second. Give them at least until mid-June to see whats going on, as well as see how House/Morales/Lipscomb/Baker do in the minors. Make a plan from there, but in all honesty, that should probably be your starting infield until you call someone up. If they hit, excellent. If not, Rizzo made sure and can comfortably move on without rushing anyone in the minors.
I don't buy much of either yours or Gata's view of how Vargas plays (I agree he's a weak shortstop defensively), but i agree he should not be starting even over a weaker internal option like Alu or Kieboom. It's tough medicine because it means a weaker team in the field, but we are still trying to find out whether we can fill positions internally. Even if Kieboom has been consistently unimpressive and Alu had a bad enough debut to make him questionable as a regular, they play (or, in Kieboom's case, perhaps get released due to lack of options) until Lipscomb / Morales / House takes over 2nd half of the year.
Not to turn this into too much of a Vargas discussion, but he hits the ball hard enough to still have a bit of upside, has great contact skills, and is not a defensive downgrade vs the alternatives at 2nd and 3rd. His average EV is something like 44th percentile (middle of the pack) and had the lowest K% of all hitters with 250 PAs not named Arraez. His launch angle is poor, but if he just elevates a bit more, and frankly, he could have a late breakout. I wouldn't count on it, he's old, but he's a fine backup, and if we want to tighten the defense for a GB pitcher, he can play either 2nd or 3rd occasionally. (BTW - tied for 8th in DRS, 13th in UZR among 3rd basemen with 300 innings).