The ValidationData in trainingOptions does not have any influence in training progress.
In actual, validation set is used to evaluate the given model and used to fine tune the model hyper-parameters. So, the validation set affects the model, but indirectly.
Also, validation of the data has no effect on training. While, the vice-versa may not be true because the model sees and learns from the training data and the validation model then evaluates the validation data. So, RMSEs of the training and validation phases have no effect on each other.
However, you can use k-fold cross validation to tune/fine-tune your parameters and get the best results for validation and test data. You can use cvpartition function for this.