Hi, & @ merkin & this is actually as per manufacturer advice on some models.
In the trade we use a manifold gauges between the vacuum pump & outdoor unit.
Otherwise there's no method of sealing the system & holding the negative pressure.
When taking the above mentioned manifold gauge lines off shredder valve an "acceptable amount of refrigerant (most likely R410a type) is lost" as outlined in koto protocol that usa championed & certainly follows
So my earlier advice of opening the liquid line first & allowing the refrigerant to purge a Small amount of vapour, the air is chased out first & the flare nut can quickly be sealed with one hand.
When taking my manifold gauges a a pipeline of 5M I'd allow -50grams of LIQUID refrigerant is lost when taking a manifold line of a fully charged system (probably 800/900 odd grams fully charged) and that's each time the engineer puts his manifolds on. I know guys who do it every 6 months as part of a manufacturer advised service/maintenance routeen
they do this to monitor pressures & record them as part of a detailed service package.
so It all depends on circumstances & if the system was expensive or not... Could have gotten it for a good price & all the "good practice" almost equals outlay on the whole system.
Us A/C engineers ain't cheep here in UK.