You could create a selection from the GeoFeatureLayer:
Dim geoFeatureLayer As IGeoFeatureLayer
Set geoFeatureLayer = featureLayer
Dim featCur As IFeatureCursor
Set featCur = geoFeatureLayer.Search(Nothing, True)
Dim feat As IFeature
Set feat = featCur.NextFeature
Dim featSel As IFeatureSelection
Set featSel = featureLayer
If featSel.SelectionSet.Count > 0 Then
featSel.Clear
End If
Do Until feat Is Nothing
featSel.Add feat
Set feat = featCur.NextFeature
Loop
Dim ss As ISelectionSet
Set ss = featSel.SelectionSet
Dim pEnumGeom As IEnumGeometry
Dim pEnumGeomBind As IEnumGeometryBind
Set pEnumGeom = New EnumFeatureGeometry
Set pEnumGeomBind = pEnumGeom
''pEnumGeomBind.BindGeometrySource Nothing, featCur
pEnumGeomBind.BindGeometrySource Nothing, ss
Dim pGeomFactory As IGeometryFactory
Set pGeomFactory = New GeometryEnvironment
Dim pGeom As IGeometry
Set pGeom = pGeomFactory.CreateGeometryFromEnumerator(pEnumGeom)
activeView.Extent = pGeom.Envelope
activeView.Refresh