So anything that causes tachycardia would?

block the vagus nerve and anything that blocks the vagus causes bradycardia. Would it be alright to assume that anything that cause brady or tachycardia either stimulates or inhibits the vagus.

no, because your heart is controlled both parasympathetically (cholinergically) and sympathetically (adrenergically). the parasympathetic and sympathetic nervous systems work independantly and oppose each other to regulate functions- the vagus nerve is the cholinergic side- stimulation slows down the heart. the adrenergic side speeds up the heart when stimulated by epinephrine or norepinephrine, and slows down when those are blocked (by beta blocking drugs like propranolol, atenolol, etc.). so to speed up the heart, you can either block the vagus nerve or increase sympathetic stimulation. to slow it down, you can either increase vagus stimulation or block sympathetic stimulation. the sympathetic response is slower in speeding/slowing your cardiac output than vagus stimulation. thats why people who have heart transplants can regulate their heart rate even though their vagus nerve was cut, it just takes their heart a few seconds longer to respond to excercise, fear, etc. than a normal person and they cant do it quite as well

I could not understand your question correctly but assuming what I could is right than yes techy or brady cardia is the stimulation or inhibition of vagus nerve respectively.