This particular line of text is probably best understood when it is written out in the line organization that Shakespeare originally wrote.
Indeed, I never shall be satisfied
With Romeo, till I behold him—dead—
Is my poor heart for a kinsman vexed.
These three lines really mess with first time readers, and I definitely have to spend some time with my classes explaining what Juliet is doing here. During Juliet's conversation with her mother in Act 3, Scene 5, Juliet is doing something very difficult with language. She's saying the words that her mother wants to hear; however, Juliet is actually saying something completely different. Juliet's words sound like she wants Romeo dead, and that is what her mother wants to hear, but Juliet is actually professing her love for Romeo. In the above few lines, the dashes are a big key to interpreting the lines. At first glance, the line could read like the following:
Indeed, I never shall be satisfied with Romeo, till I behold him dead.
Lady Capulet hears Juliet saying that holding Romeo's dead body will make her happy; however, that ever so subtle dash/pause makes it possible to read "dead" with the second half of the text. It would read like the following:
Dead is my poor heart for a kinsman vexed.
That has an entirely different meaning. Juliet is essentially saying that her heart is dead over Tybalt's death. The lines could also be interpreted as Juliet saying that she won't be satisfied with Romeo until the two of them have lived a long and loving life, and he dies in her arms. These lines are not the only lines in this scene that have hidden meanings. Juliet asks for the poison so that she may "temper" it and use it against Romeo. Lady Capulet thinks that Juliet wants to fiddle with the poison to make it more effective, but Juliet wants to "temper" it and make it weaker. Juliet ends her tricky speech with the following lines.
Oh, how my heart abhors
To hear him named, and cannot come to him.
To wreak the love I bore my cousin
Upon his body that slaughtered him!
She is saying that she can't stand Romeo's name, which is what her mom wants to hear; however, Juliet can't stand to hear it because she can't be with him. Lady Capulet also hears that Juliet wants to "wreak" the love that she once had for Tybalt upon the man that killed Tybalt. Lady Capulet would think that Juliet wants to take her emotions out on Romeo, but Shakespeare's audience would fully understand by now that Juliet is dropping a huge innuendo about wreaking her love upon Romeo's body.