It sounds like the part you're asking about is part 22 in this diagram.
Buy OEM Parts for Suzuki Motorcycle 1997 Cylinder Head Diagram
Do you absolutely need to replace it? I guess that depends on what condition it's in. Carefully inspect it and if it doesn't look worn, maybe you can use it again. If you choose to re-use it, make sure you test for vacuum leaks after putting it all back together, and repeat the test at a regular interval (perhaps when you're doing chain maintenance or oil changes).
Then again, the advice I've heard from Yermo
again and again is, "if there's any doubt; there isn't any." Or, if you're not sure if you need to replace it, it means you need to replace it. Luckily the part is only $2.49 at Partzilla.
See also this manual, which recommends replacing the o-ring as well. https://www.carlsalter.com/mcpdf/Suzuki_GSF600_95-99.pdf
Think about it, if those boots are old and cracked and need to be replaced, when was the last time you think the o-ring was replaced? How old and worn out must it be? Even if you make it work this time, you're probably just buying yourself a little bit of time before you have to do the work all over again to replace a leaking o-ring. Better to just get it over with now and not have to worry about it again.
When I got my Bandit, we spent a few months going over it and I think we replaced every piece of rubber on the bike. It was all old and deteriorating.