Is there a way to insert an effect using SONAR Platinum?  I want to insert an echo effect on a vocal track at a particular bar for 4 - 6 beats.

The easiest way to do this is to just insert the effect in the FX bin of the track (or an FX Chain in the ProChannel, and then use an automation envelope to turn the effect on during the required bars. You'll have to read up on how to draw automation to do this. But it's the most versatile way of triggering effects on specific bars. 

Another option is to clone the track and delete the clips from the clone. Splitting clips from the original track you can shift-drag the ones for special effects to the clone (shift-drag locks time). Depending on the effect and how long the tail is, this has been simpler for me (also easier to find and tweak later on).

Or add to the clip's own FX bin.

Usually I automate delay's send AND return (volume) to achieve desired sound, it gives more possibilities. You control amount and timing of signal you send to echo, and then you control volume of echo (and feedback, and other options) separately.

KPerry
Or add to the clip's own FX bin.

Good idea but one drawback - any tails (especially with Delay) will be chopped off once the Now time reaches the end point of the clip.
 
I would go with the solution offered by mettelus - clone the track, isolate the clip(s) you want the effect added to and delete the rest.
Now you put your Delay effect in the Track Fx bin with no worries about tails.
 
Remember to mute (or delete) the corresponding clip(s) in the main track

Well, there is another way - use sends and automate sends or destination bus.

Bristol_Jonesey

KPerry
Or add to the clip's own FX bin.

Good idea but one drawback - any tails (especially with Delay) will be chopped off once the Now time reaches the end point of the clip.

I just create another Take Lane in the same Track, put the clip there, and extend the end with silence.

So many different ways to do the same thing

I borrowed an idea from a live setup.  I wanted to use a fader on my control surface to control the feed as opposed to using a mouse.... I want to mix in real time as mix as possible... and clean it up after - the same way you might with midi.
 
In this session in particular - I Created 2 busses in sonar - 1 is title delay feed - the other is titled Delay.   The delay buss had an eq - comp - delay - de esser - delay - and a post fader send to the main verb.  in that order -
 
I sent lead vocals, Lead guitar to the Delay Feed - as - in this particular track - they both would benefit from this.  Now - I could use a longer throw fader to be more expressive with my delay feeds - catching just a word... or sneaking up the delay on long notes... or long words... and feeling like I wesnt just being clinical in using a mouse.
 
I also found that I could be more musical - even if I automated the delay feed with a mouse in real time - it has a much larger and easier to understand footprint than a rotary encoder.
 
So now I've automated my delay feeds coming in - I can easily control the return volume and eq in its own bus