Ok, as to why ":vscmd Smartcomplete" doesn't work, that one is easy: you need to provide a full command, the type of commands that work in the QuickFind combo box when preceded with '>' (this is not too well known, but it works). Thus, you need to use ":vscmd Macros.Smartcomplete" and that should work fine. You can also use the ViEmu shortcut ":macro", which prepends the "Macros." itself, so ":macro Smartcomplete" should work.
Regarding he fact that "Tab" by itself does nothing, how have you bound it to the macro? I'm not sure VS allows binding Tab (at least it doesn't allow me to enter it in Tools|Options|Keyboard). Knowing how you have bound it should help.
As to the autocompletion box not paying attention to input, it's normal that does happen if ViEmu is still in normal mode - autocompletion needs ViEmu to pass keypresses through, and it only does so in insert mode. If you're binding <tab> through ViEmu, it would definitely pay off to have an 'i' after ":macro ...<return>", to make ViEmu pass input through. Hopefully that will fix it.
Finally, regarding why the Tab is inserted in the wrong place, I'd need more details about how you have bound it and I'll try to understand what is happening.
VS<->ViEmu integration is pretty complex, as VS is not designed for such a radical change, and I handle all built-in cases in the code, but it's possibly necessary to do something more to do what you are trying to.