  We have studied extracting $|\vub/\vcb|$ by calculating the ratios ${\cal
B}(\bar{B}^0\to D_s^{(*)-}(\pi^+,\rho^+))/{\cal B}(\bar{B}^0\to
D_s^{(*)-}D^{(*)+})$ including penguin effects within the factorization
assumption. The ratios involving $\bar{B}^0\to D_s^-D^{+}$ mode have
considerable penguin corrections ($\sim 15%$ at the amplitude level), but those
involving $\bar{B}^0\to D_s^-D^{*+}$ mode have relatively small penguin
corrections. On the other hand, the $\bar{B}^0\to D_s^-D^{+}$ mode has smaller
form-factor dependance. Therefore, these ratios complement each other in
measuring $\vub/\vcb$. The theoretical uncertainty from the hadronic form
factors in our method is at the level of 15%, which is comparable to the
model-dependence uncertainty of about 20% in the measurement of $| \vub/\vcb |$
from the exclusive semileptonic B decays. Using the newest upper limit on $B\to
D_s \pi$ decay from CLEO, our method sets an upper limit $| \vub/\vcb | < 0.13$
which is very close to the measured values from the semileptonic B decays. We
also discuss the possible breaking of factorization assumption.
