Portia makes a speech to Bassanio in which she tells him that he is now master of the mansion and of herself. She also hands a ring to Bassanio:
I give them with this ring, Which when you part from, lose, or give away, Let it presage the ruin of your love,
And be my vantage to exclaim on you.
(171-74)
The fairy-tale quality of the subplot thus continues with this ring business. The ring becomes a magical symbol of their love, and the loss of the ring would thus signify an end of their love.
Bassanio puts the ring on his finger and makes a solemn vow or promise that he shall never remove the ring:
But when this ring Parts from this finger, then parts life from hence.
(183-84)
Bassanio claims that the onlywaythat the ring will ever leave his finger is when he is dead and thus unable to stop anyone from removing it. The vow is a vow of honor, and Bassanio must keep his word. However, this is a comedy, not a fairy tale. And so the ensuing events will not be the same as those in a fairytale.