We are using seattle.master.
The local navigation is on LHS of page.
The global navigation is at top of page.
Is it possible to place the global navigation on the LHS (vertically) instead of the top and remove the local navigation?
Everything is possible! If you have a custom master page you can move around the ContentPlaceHolders as you wish (even empty some of them).
But, you will probably need to write a lot of css to make it look good, and also, the left navigation can be overwritten by page layouts so if you choose this path you should probably make your own page layouts as well and limit your users to those page layouts.
Seems to me like lot of work for an unclear purpose. Maybe you should write a custom current navigation (left nav) instead?