vergeblich
futile
Without the desired success or hoped-for result; in vain.
All seine Bemühungen, sie umzustimmen, waren vergeblich.
All his efforts to change her mind were futile.

futile
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futile
Without the desired success or hoped-for result; in vain.
All seine Bemühungen, sie umzustimmen, waren vergeblich.
All his efforts to change her mind were futile.

'Vergeblich' (futile/in vain) is very often used as an adverb to describe an action that remains unsuccessful. In that case, it usually comes directly after the conjugated verb.
Ich habe vergeblich versucht, ihn zu erreichen.
I tried in vain to reach him.
As an adverb, 'vergeblich' (in vain) often appears together with verbs that express an effort or expectation. Typical combinations are 'vergeblich warten' (to wait in vain), 'vergeblich suchen' (to search in vain), and 'vergeblich hoffen' (to hope in vain).
Wir warteten vergeblich auf eine Antwort.
We waited in vain for an answer.
You can also use 'vergeblich' (futile) with the verb 'sein' (to be) to express that an effort or attempt was in vain. The structure is then: 'Etwas ist/war vergeblich' (something is/was futile).
Seine ganze Mühe war am Ende vergeblich.
All his effort was futile in the end.
successful
'erfolgreich' and 'vergeblich' are clear opposites. You use 'erfolgreich' when a goal is reached or a result is good. You use 'vergeblich' when someone makes an effort but does not achieve the desired result. Both words often fit attempts, plans, or work. The difference is simple: with 'erfolgreich', it works out; with 'vergeblich', it does not work out despite the effort.

Tom hat vergeblich versucht, den alten Drucker zu reparieren.
Tom tried in vain to repair the old printer.
Die Suche nach einer neuen Wohnung war endlich erfolgreich.
The search for a new apartment was finally successful.