Substitute while loop with functional code

I am refactoring some scala code and I am having problems with a while loop. The old code was:

for (s <- sentences){
// ...
   while (/*Some condition*/){
      // ...
      function(trees, ...)
   }
}

I've have translated that code into this one, using foldLeft to transverse sentences:

sentences./:(initialSeed){
 (seed, s) =>
     // ...
     // Here I've replaced the while with other foldleft 
     trees./:(seed){
         (v, n) =>
             // ....
             val updatedVariable = function(...., v)
     }
}

Now, It may be the case that I need to stop transversing trees (The inner foldLeft before it is transverse entirely, for that I've found this question:

Abort early in a fold

But I also have the following problem:

As I transverse trees, I need to accumulate values to the variable v, function takes v and returns an updated v, called here updatedVariable. The problem is that I have the feeling that this is not a proper way of coding this functionality.

Could you recommended me a functional/immutable way of doing this?

NOTE: I've simplified the code to show the actual problem, the complete code is this:

val trainVocabulart = sentences./:(Vocabulary()){
  (vocab, s) =>
    var trees = s.tree
    var i = 0
    var noConstruction = false

    trees./:(vocab){
      (v, n) =>
        if (i == trees.size - 1) {
          if (noConstruction) return v
          noConstruction = true
          i = 0
        } else {
          // Build vocabulary
          val updatedVocab = buildVocabulary(trees, v, i, Config.LeftCtx, Config.RightCtx)

          val y = estimateTrainAction(trees, i)

          val (newI, newTrees) = takeAction(trees, i, y)

          i = newI
          trees = newTrees

          // Execute the action and modify the trees
          if (y != Shift)
            noConstruction = false

          Vocabulary(v.positionVocab ++ updatedVocab.positionVocab,
            v.positionTag ++ updatedVocab.positionTag,
            v.chLVocab ++ updatedVocab.chLVocab,
            v.chLTag ++ updatedVocab.chLTag,
            v.chRVocab ++ updatedVocab.chRVocab,
            v.chRTag ++ updatedVocab.chRTag)
        }
      v
    }
}

And the old one:

for (s <- sentences) {
  var trees = s.tree
  var i = 0
  var noConstruction = false
  var exit = false
  while (trees.nonEmpty && !exit) {
    if (i == trees.size - 1) {
      if (noConstruction) exit = true
      noConstruction = true
      i = 0
    } else {
      // Build vocabulary
      buildVocabulary(trees, i, LeftCtx, RightCtx)

      val y = estimateTrainAction(trees, i)

      val (newI, newTrees) = takeAction(trees, i, y)

      i = newI
      trees = newTrees

      // Execute the action and modify the trees
      if (y != Shift)
        noConstruction = false
    }
  }
}