Here's a link to an article about a couple that took their Vega around the world and what they changed and upgraded.
http://americanvega.org/images/Tarka_the_Otter.pdf
Here's a link to various Vega 27 voyages also with upgrade information.
Long and Short Voyages of Vega 27 sailboats and others
I think I would certainly inspect the bulkheads and maybe add tabbing if I deemed it necessary but not double them. Everything obviously needs to be inspected but I wouldn't build a second boat inside the first - the Vega is a strong boat that has proven itself many times over. These items are more labor intensive than dollar intensive. Chainplates need to be checked and rreplaced if needed. The ports are large but storm shutters are easy to fabricate. The Vega has pretty simple systems so unless you're adding a lot of electrical equipment the rewiring won't be too expensive.
Watertight bulkheads are hard to do effectively but here's a link to James Baldwin's site.
Atom Voyages | About the Sailboat Atom
His Atom (Pearson Triton) has taken him around the world and he gives good advice on changes necessary as well as water tight bulkheads.
Brian