The best replacement for vsto is vba.

The best alternatives to vsto are rated as following:
★★★★★  vba
★★★  interop
★★  exceldna
★★  openxml
★★  vba excel
★  vsta
★  xll
★  addin
★  add in express
  netoffice

Your alternative for vsto is not listed? Suggest!
Speaking about vsto, one probably shouldn't forget about clickonce.

Other search terms related to vsto and their ratings are:
★★★★★ clickonce
You have in mind a companion for vsto and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of vsto in all years. Click on graph for more.
vsto
vba
interop