I can feel the rising and falling of sensations in vipassana, but what to make of thoughts? There are two things I want to understand here. Firstly, though I'm not overly disturbed by thoughts during vipassana sitting, I just don't know how do I treat them. What I presently do is just try to ignore them and continue observing the rising and falling. But if the thoughts are too strong and persist, I tend to observe them impartially until they leave of their own accord (but during this time my observance of anicca has stopped). Am I doing this thing rightly or not?

Secondly, I want to understand the reality of thoughts. I know that ultimately none of these have permanent reality, but compared to conditioned reality of sensations what is the conditioned reality of thoughts? Basically, I want to know whether they have any objective reality, or they are subjective (just things in my head)?

I recently read in a book that reality keeps vibrating each and every instant, and we need to look into the arising/vanishing of these vibrations, as that is what anicca is all about. My question is do these vibrations apply to thoughts as well or just sensations?
How should I treat thoughts?
You should strive to know thoughts as thoughts. If you are observing impartially, that's good. If you are ignoring the light, fleeting thoughts, it depends what you mean. You don't want to actively ignore anything as that will cultivate aversion or even a dullness of mind. If you mean the thoughts are just too light, too fleeting to catch, then you're doing what you can, and your coverage will improve with practice, if you strive for it.
The other side of this is that it's more important to impartially observe whatever comes up, rather than simply impartially observing thoughts. That is, you should try to stay in the here and the now. A thought might become frustration, and you should observe that impartially. Then the frustration becomes hotness in the body. As soon as we are aware of the hotness, we should observe the hotness. Or perhaps coldness following fear. And then maybe the thought comes back, and we observe that, or we feel an irritation on the skin, and we observe that. I'm giving as many examples as I can from my own experience to try to illustrate that when a thought overtakes us, the here-and-now can be a swiftly-moving target.
What is the conditioned reality of thoughts?
The Buddha placed thoughts on an equal footing with all other sensations, as you can read in the very brief Sabba Sutta (found in the Samyutta Nikaya, Sutta #35.23):

&quot;Monks, I will teach you the All. Listen &amp; pay close attention. I will speak.&quot;
&quot;As you say, lord,&quot; the monks responded.
The Blessed One said, &quot;What is the All? Simply the eye &amp; forms, ear &amp; sounds, nose &amp; aromas, tongue &amp; flavors, body &amp; tactile sensations, intellect &amp; ideas. This, monks, is called the All. Anyone who would say, 'Repudiating this All, I will describe another,' if questioned on what exactly might be the grounds for his statement, would be unable to explain, and furthermore, would be put to grief. Why? Because it lies beyond range.&quot;

So from a meditative point of view, the most objective things you have are the thoughts and the senses. Don't worry too much whether thoughts are objective or subjective. Rather consider objectively that a thought is a thought, and seeing it as anything else is subjective.