If the keyboard has not been damaged (say by liquid spills) the most likely cause for many of the above reports is one of:
- the keyboard cable not making good contact with the connector
- a tiny break in one or more of the conductors in the cable
- a tiny break in one or more traces on the keyboard's circuit board
All of these can easily occur with age, be intermittent, respond to pressure or to changes in temperature, etc.
If reseating the connector for the keyboard cable doesn't fix it, your best bet is to replace the keyboard. They're cheap and this is an easy DIY job. Don't mess around putting pieces of paper in the machine, etc.
I've bought several replacement keyboards for Dell laptops (I've been using them since the C-series) on eBay with 100% success. Searching eBay for these words:
Dell Adamo 13 keyboard
brings up many hits for new keyboards in the $10-$30 range, almost all from sellers with very high feedback ratings. And it's an easy swap. The Dell web site in the "support" pages has a service manual for your Vostro with R&R instructions for all field-swappable components, including the keyboard. Go for it :)
Disclaimer: No affiliation with Dell or eBay except as a satisfied customer of each.