I reckon your translation is good (where non-classical vane blanditie = vanae blanditiae, as you say). Note "sciunt" can mean "they are assured that" as in "they can be assured that"
Bona mihi equidem videtur tua versio. Habes etiam anglicè "be assured that" pro "scito" verbo.
so "No one can as you truly can, my king, because empty compliments escape you [i.e., you don't deserve them] or they are [/can be] assured that you are at hand [/standing by] or they fear if [/whether] you were to know [/get wind] or would perhaps come..."
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.