I have a Harby with Sentinel BUK, but I'm really starting to wonder (AGAIN) if I made the right choice, hahaha. Should I instead have a Sentinel with Harby BUK?
The Sentinel has much less armor and shields than the Harby, but it also has about half the EM signature with everything turned on 40k v 22k. It appears someone input the wrong reactors in the Hardpoint for the Harby and Sentinel, because the Sentinel's GammaMax dumps 3,866 EM for a ship total of 6,052, while the Harby's TurboDrive only emits 1,042 for a ship total of 1,092. Obviously something is wrong here. I think whomever input the numbers has the Harby and Sentinel backward.
When you scan the Harby reactors they start around 1k and actually go down from there. So what I did to test this is I put the same reactor in both frames. In the Harby, the SparkJet emits a mere 926 em. In the Sentinel, it emits 3,436. This appears to me to suggest that one of them has a better box around the reactor, shielding the EM from escape, such as we see in all true stealth frames. I think they just input these two backward.
Take away for the time being is that they intend the Sentinel to be a real stealth ship. If you intend to sneak around and fire missiles from a distance, you'll likely want the stealth frame. If you're not going to do that you probably want the sturdier Hoplite frame. It looks like the Sentinel does come with both a reactor box and stealth armor.
My personal opinion is that if these numbers tell us anything useful (which they may not) it does appear the Vanguard can be run silently when turning off everything but one reactor, and you can choose that reactor to be as low emission as possible (with the other one very high). The difference between 1k emission and 6k is huge. 1k is probably low enough to get inside the Arrestor III's 9k targeting range against most small and medium radar, given the impunity the Eclipse seems to have against both those classes.
The Harby's S5 missiles target from 15-20k. You might be able to launch those against large and cap radar targets without detection, if you have the Sentinel's stealth frame.
When we think about how to use these ships we most often consider how we would use it alone, but I would urge you the most powerful use of the Vanguard is going to be in strike teams made up exclusively of Vanguard, but including several types. So if you consider one task and feel one way about it, and then consider another task and feel differently, could be all you're identifying is the need for teamwork.
But given this, right now I would take the Sentinel with Harby and Hoplite BUKs, since I like the stealth role.