Interference from another meditation technique in my vipassana meditation In my 20's(I am now 55) I practiced light and sound meditation for 7 years. When I am doing vipassana which I started in March this year (having not meditated for 25 years) I get the 'sound' mainly engulfing me and it is very blissful I must say. I might also get a huge gushing out of the top of my head and that draws me in to. These moments can feel very blissful. 
I made a conscious effort to ignore these events and have mainly succeeded but sometimes I still get sucked in because I want to or as a 'treat'. 
What effect will this have on my vipassana? Is it safe to allot some time for that meditation outside of the vipassana meditation? Is it ok toward the end of a sitting to let myself focus on the sound instead? Should I let the sound buzz through me at the same time I focus on my body parts?
You do not go into any detail about what vipassana you are practicing but with the information on hand the following advice:

In insight meditation (vipassana) ALL phenomena have the following qualities:


Anicca - they are impermanent.
Anatta - they are not-self.
Dukkha - that if we cling or have aversion to these impermanent phenomena this creates suffereing.


You 'sound' experience is fairly common, I have it as a constant meditation companion in some form or other, but it falls into the same category of any other phenomena.


  These moments can feel very blissful. I made a conscious effort to ignore these events and have mainly succeeded but sometimes I still get sucked in because I want to or as a 'treat'.


There is both clinging and aversion here. Do neither (push away or hold onto) - just view both the bliss and the sound with equanimity. Equanimity is a core skill you are learning in any vipassana practice. So let go of the need to 'control' any of this and just try to see that all these phenomena have the same intrinsic nature as a 'thought', a body sensation, a physical sound. They are all phenomena with the same above three characteristics.