Hey guys. I just bought a Vibrant today and am having this same issue. No matter how many times I hold either volume button independently (or even together), I can't get into Fastboot, or I guess Download mode as it's called here. I'm coming from a Nexus One where it works everytime, so this is kind of weird. Launching RomManager and selecting to boot into recovery does work, but I don't want to rely on that.
Is the smart thing to go swap the physical phone?
...Galaxy Nexus - rooted running Bugless Beast ROM
...HTC Sensation 4G - rooted, all stock, 2.3.4
...LG G2x - temp rooted, testing CM7 nightly (sadly retired)
..Samsung Nexus S - rooted and on latest CM7 build. (retired)
..MyTouch 4G, now with permaroot! (retired)
..Samsung Vibrant w/ root access & clockworkmod recovery (retired)
..Nexus One w/ locked bootloader and latest nightly CM6 build (retired)
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