If your layout only has one viewport, you should be able to simply just toggle Model and Paper space using the SwitchToPaperSpace and SwitchToModelSpace functions.
https://help.autodesk.com/view/OARX/2020/ENU/?guid=GUID-F0906774-90AD-4AD2-9894-E37ACEF8035A
If you want to get the viewports on the current layout, you can get the Block Table Record associated with the layout and then iterate its objects looking for the viewports. Once you find the viewport, then you can set it current, switch to model space, zoom, and then switch back to paper space.
' Step through current layout
<CommandMethod("StepLayout")>
Public Shared Sub StepLayout()
' Get the current document and database, and start a transaction
Dim acDoc As Document = Application.DocumentManager.MdiActiveDocument
Dim acCurDb As Database = acDoc.Database
Using acTrans As Transaction = acCurDb.TransactionManager.StartTransaction()
' Reference the Layout Manager
Dim acLayoutMgr As LayoutManager = LayoutManager.Current
' Get the current layout and output its name in the Command Line window
Dim acLayout As Layout =
acTrans.GetObject(acLayoutMgr.GetLayoutId(acLayoutMgr.CurrentLayout),
OpenMode.ForRead)
' Open the Block table for read
Dim acBlkTbl As BlockTable
acBlkTbl = acTrans.GetObject(acCurDb.BlockTableId, OpenMode.ForRead)
' Open the Block table record for the current layout
Dim acBlkTblRec As BlockTableRecord
acBlkTblRec = acTrans.GetObject(acLayout.BlockTableRecordId,
OpenMode.ForRead)
' Step through the Block table record
For Each acObjId As ObjectId In acBlkTblRec
acDoc.Editor.WriteMessage(vbLf & "DXF name: " & acObjId.ObjectClass().DxfName)
acDoc.Editor.WriteMessage(vbLf & "ObjectID: " & acObjId.ToString())
acDoc.Editor.WriteMessage(vbLf & "Handle: " & acObjId.Handle.ToString())
acDoc.Editor.WriteMessage(vbLf)
Next
' Dispose of the transaction
End Using
End Sub