A coherent vibrational wavepacket is launched and manipulated in the symmetric stretch (a1) mode of CBr4, by impulsive stimulated Raman scattering (ISRS) from nonresonant 400 nm laser pump pulses with various peak intensities on the order of tens of 1012 W/cm2. Extreme ultraviolet (XUV) attosecond transient absorption spectroscopy (ATAS) records the wavepacket dynamics as temporal oscillations in XUV absorption energy at the bromine M4,5 3d3/2,5/2 edges around 70 eV. The results are augmented by nuclear time-dependent Schrödinger equation simulations. Slopes of the (Br 3d3/2,5/2)-110a1* core-excited state potential energy surface (PES) along the a1 mode are calculated to be -9.4 eV/Å from restricted open-shell Kohn-Sham calculations. Using analytical relations derived for the small-displacement limit and the calculated slopes of the core-excited state PES, a deeper insight into the vibrational dynamics is obtained by retrieving the experimental excursion amplitude of the vibrational wavepacket and the amount of population transferred to the vibrational first-excited state as a function of pump-pulse peak intensity. Experimentally, the results show that XUV ATAS is capable of resolving oscillations in the XUV absorption energy on the order of a few to tens of meV with tens of femtosecond time precision. This corresponds to change in C-Br bond length on the order of 10-4 to 10-3 Å. The results and the analytic relationships offer a clear physical picture, on multiple levels of understanding, of how the pump-pulse peak intensity controls the vibrational dynamics launched by nonresonant ISRS in the small-displacement limit.