Try below code.
Its working and to change the textbox properties(autocompletemode and autocompletesource)
Cn.Open()
Dim dt As New DataTable
Dim da As New OleDbDataAdapter("select ...........", Cn)
da.Fill(dt)
Dim row As DataRow
txtvendor.AutoCompleteCustomSource.Clear()
For Each row In dt.Rows
txtvendor.AutoCompleteCustomSource.Add(row.Item(0).ToString())
Next