Risolto!
Scrivo la soluzione in caso qualcuno dovesse usare lo stesso componente.
Public Sub Page_Load3(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim camp, lab As String
Dim sql As String
lab = Session.Item("idlaboratorio")
camp = Session.Item("idcamp")
Dim y As String = lab.Trim
Dim x As String = camp.Trim
sql = "PARAMETERS [y] String " + lab.Trim + ",[x] String " + camp.Trim + "; SELECT * from Dati_campione WHERE FK_idlab='" + [y] + "' AND FK_idcamp='" + [x] + "'"
C1WebReport1.ReportSource.FileName = "lucky.xml"
C1WebReport1.ReportSource.ReportName = "Dati_Campione Report"
C1WebReport1.Report.DataSource.RecordSource = sql
C1WebReport1.ShowPDF(True)
End Sub