I have a portable Tascam DP 24 recorder, with 8 inputs at once, and would like to record a demo band in a practice studio and later mix it with Sonar. Here's my setup :
 
3 mic for drums (one kick, 2 overheads), direct to Tascam DP.
1 mic for guitar amp, direct to Tascam DP.
1 DI for bass guitar (XLR output goes direct to Tascam DP, link to bass amp)
2 line-in for keyboard (direct to Tascam DP, the keyboard has additional outputs for keyboard amp)
1 vocal <-?
 
I've done the recording before using the same setup with another band but without vocal and works fine.
 
With the vocal, I'm thinking of :
 
a) Using a direct box on a condenser vocal mic, the XLR output goes to the Tascam, link goes to the PA system mixer. 
..or should it be reversed : link (line) goes to Tascam, XLR goes to PA system mixer, or it doesn't matter ?
 
b) Condenser vocal mic goes to the PA system mixer, and send an AUX output (line) to Tascam.
 
..or, any of the above method would works the same ?
 
Thanks!

A condenser mic is not your best choice for this situation. A condenser mic needs phantom power to operate and unless your direct box generates phantom power your mic wont work, except in the instance where the "link' is an XLR that is directly linked to the XLR input. This applies to both passive and active DI's.   
 
In this instance you could plug into either, as long as one was receiving phantom power from either the PA or the Tascam. The output of the direct box would then drive whichever device wasn't supplying phantom power.
 
To be effective your DI must have an XLR 'link'. However, if the 'link' is buffered, then no go. The majority of DI's have just a 1/4" TS jack as the 'link', which wont get phantom power to you mic.
 
It is much simpler to use a dynamic mic.
 
You could use option (b, but you need to decide if your signal out needs to be pre or post eq.
If you have inserts on the PA channel strips, they normally are pre eq and can be used to drive the Tascam. You can then use your condenser mic if the PA provides phantom power.

Using a dynamic microphone would indeed simplify things. If you do end up using a dynamic instead of a condenser, some DI boxes (like this) can serve as a splitter, with an XLR to 1/4" adapter. There are also devices called "splitters" for just your application. Go to Sweetwater.com and enter "splitter" into the search box.

Klark Teknik DN1248

Thanks for all the suggestions!
 
I ended up using a dynamic mic, feed to the mixer and route the aux (pre eq) to the Tascam and it's a simple routing.
 
I'll look into the splitters too. I've had the Behringer Ultra-DI - which can supply phantom power, and while the instrument input works, the mic XLR is not passing any signal through (static noise instead).. I suspect I might have damaged it after dropping it once.