How to switch off validation for ajax call?

Hi Ivy Team

I have a autocomplete. The autocomplete is required field. In autocomplete. I have ajax to handle change event. When autocomplete changes, some fields will be updated below. But When I clear data from autocomplete. The message required is display and some fields are no more update. So I want to do validation only when a button is clicked which submits the form but not when autocomplete changes. How I can do this?

Note: I try using "immediate" attribute in ajax to switch off validation but it is not work.

<p:autoComplete id="select-vendor" 
                widgetVar="select-vendor"
                value="#{purchase.vendor}" 
                dropdown="true" 
                scrollHeight="400"
                completeMethod="#{tenderInformationBean.completeVendor}"
                var="vendor" 
                itemLabel="#{vendor.vendorName}" 
                itemValue="#{vendor}" 
                maxlength="50"
                converter="pojoConverter" 
                cache="true" 
                queryDelay="500"
                rendered="#{!tenderInformationBean.isVendorNotFound}" 
                binding="#{tenderInformationBean.vendorNameAutoComplete}"
                disabled="#{isReadOnlySupplier}" 
                forceSelection="true"
                requiredMessage="#{ivy.cms.co('/ch.ivy.addon.portalkit.ui.jsf/common/requiredFieldMessage')}" >

                <p:ajax event="change" update="vendor-information-fieldset" global="false"
                    listener="#{tenderInformationBean.onSelectVendorName(purchase)}" onstart="if(PF('select-vendor').panel.is(':visible')) return false;"
                    immediate="true"/>

                <p:ajax event="itemSelect" update="vendor-information-fieldset" global="false"
                    listener="#{tenderInformationBean.onSelectVendorName(purchase)}" />

                <f:validator validatorId="awsValidator" />
            </p:autoComplete>

Thanks