vouch[vautʃ] 来自拉丁词根vot,vou,vow，表示断言，愿望或者庄严承诺。跟voice有关

to say that you firmly believe that something is true or good because of your experience or knowledge of it
to say that you believe that someone will behave well and that you will be responsible for their behaviour, actions etc
vt. 担保；证明；传出庭作证
vi. 保证；证明；确定

voucher['vautʃə]]
an act of vouching
a piece of supporting evidence
a coupon issued by government to a parent or guardian to be used to fund a child's education in either a public or private school
a piece of paper that can be used to pay for particular goods or services or that allows you to pay less than the usual price for them
n. 收据；证人；保证人；证明者；教育补助金券；优惠凭证，购物优待券

例句
I've read this report carefully and I can vouch for its truthfulness.
我已仔细看过这份报告，我可以保证它的真实性。
He vouched that she was really with him at the time.
他保证当时她确实和他在一起。

vow [vau]
to make a determined decision or promise to do something
a serious promise or decision
n. 发誓；誓言；许愿
vt. 发誓；郑重宣告
vi. 发誓；郑重宣告

词组短语
exchange vows订婚；结婚
under a vow发过誓 ,许愿

例句
We made a vow to stay friends forever.
我们发誓永远做朋友。
My parents are under a vow to go to church every Sunday.
我父母许愿,每星期日都去做礼拜。
She vowed that she would take the matter to court.
她信誓旦旦地说一定要把此事告到法庭。

avow[ə'vau]
to state or admit something
(formal) to say firmly and often publicly what your opinion is, what you think is true, etc
To state positively
to declare openly, bluntly, and without shame
vt. 断言；承认；公开宣称；坦率承认；自称;自认

例句
He avowed himself an opponent of all alliances.
他声称他是所有联盟的反对者。
He avowed that he was guilty.
他坦率承认自己有罪。
He avowed himself a patriot.
他自称自己是爱国者。

avowal[ə'vauəl]
n. 声明，公开表示；公开宣布

disavow[,disə'vau]
to say that you know nothing about something, or that you have no responsibility for or connection with something:
vt. 否认，否定；抵赖；拒绝对…的责任

例句
They publicly disavowed any connection with terrorist groups.
他们公然否认与恐怖集团有任何联系。