Find values in Dataset that match values in a Datatable

I am working on some code in a winforms vb.net (2.0) project. I have a CSV file loaded into a Datatable, and some user data loaded from a MySQL SQL query into a Dataset.

The CSV Data (in the Datatable) contains userid,date_completed

The data in the Dataset is userid,status,dateupdated

What I want to do is find any instances in the datatable where the userid matches a userid in the dataset and if so, update the dateupdated in the Dataset with the Date_completed in the Datatable.

I have tried doing this using code but it is really slow.

Does anyone know of a quick way, using vb.net code (or c# i can convert) to achieve this quickly?

Thanks